*** AMPL change log and recent bug fixes ***

Below is a summary of AMPL changes from 30 Nov. 1992,
including a summary of recent bug fixes.  For a much more complete
list of bug fixes, send netlib@netlib.bell-labs.com the E-mail message

	send fixlog from ampl

or otherwise obtain /netlib/ampl/fixlog, which is available at
various URLs, such as

	ftp://netlib.bell-labs.com/netlib/ampl/fixlog.gz
	http://netlib.bell-labs.com/netlib/ampl/fixlog.gz
	http://www.netlib.org/ampl/fixlog

Note that AMPL versions are strings of the form yyyymmdd, giving
the date (year = yyyy, month = mm, date = dd) of the last change
affecting the version.  You can determine the version of your
"ampl" program by issuing the AMPL command

	option version;

or by invoking ampl with the -vv option.  To see the version
without executing ampl further, use the invocation

	ampl -vvq

An up-to-date summary of AMPL changes and recent bug fixes is available
by electronic mail: send netlib@netlib.bell-labs.com the E-mail message

	send changes from ampl

Here is a summary of changes and recent bug repairs.  Each block of
changes is preceded by the version (yyyymmdd) in which the changes
first appeared.

19921218:
  Add initial version of "show" command:
	        "show;" or "show >filename;"
	lists all model entities.
	        "show name;" or "show name >filename;"
	shows name's declaration if it has one, or else
	lists model entities of the kind indicated by the
	first letters of name:
	        ch... ==> checks        c... ==> constraints
	        f...  ==> functions     o... ==> objectives
	        p...  ==> parameters    s... ==> sets
	        v...  ==> variables


19930208:
  Add new option relax_integrality:
	option relax_integrality 1;
causes "integer" and "binary" attributes of variables to be ignored
(in solve and write commands).

19930303:
  Variable.lb3, .ub3 withdrawn.
  Add logic and option constraint_drop_tol (default 0) to deal with
subtle presolve bug apparently caused by roundoff error:  with $presolve
> 1 and $var_bounds == 1 (the defaults), constraint bounds were very
occasionally relaxed due to bounds only conveyed for $var_bounds > 1;
this could increase the size of the feasible region, possibly making the
problem unbounded.  (Only known example:  test problem MAROS from
netlib's lp/data.)  The fix involves keeping two sets of constraint
bounds and switching between them based on $var_bounds.  The constraint
bounds for $var_bounds == 1 are only relaxed if roundoff poses no danger
or the deduced bounds on the constraint body are sharper than the
declared bounds by at least $constraint_drop_tol.  (The default
$constraint_drop_tol value 0 causes both sets of constraint bounds to be
the same and gives the same behavior as before this change.)  New
constraint dot values:  constraint.lbs1, .ubs1, lbs2, .ubs2 = versions
of .lbs, .ubs corresponding to $var_bounds <= 1 or > 1, respectively.
Constraint.lbs, .ubs still reflect the bounds sent to the solver.
  Delete warning about possibly incorrect dual values inferred for
constraints eliminated by presolve: the inferred dual values are
now believed to be correct in all cases.

19930315:
  Adjust presolve to use directed roundings on (some) IEEE-arithmetic
machines.  This sometimes leads to fewer surprises, such as an
unbounded objective under default conditions with netlib's "maros
from lp/data".

19930506:
  New command-line option -v2 merges stderr with stdout both for
ampl and for subprocesses (whereas -ve just merges them for ampl).
  Allow negative precision with %f, with formatting as for the
print command with $print_round < 0.

19930515:
  For entities with several subscripts, enforce the rule that literals
must be quoted in the model.  The first printing of thet AMPL book
(middle of p. 252) erroneously shows some unquoted literals, which the
AMPL translator should not have accepted.

19930615:
  Use stronger deduced variable bounds rather than declared variable
bounds in deciding whether an integer variable is binary.

19930208): option presolve 0 ignored integrality.
  Adjust .nl files to convey (in their first 10 lines) counts of
nonlinear integer variables and of nonlinear variables used in both
constraints and objectives (nlvb).  When nlvo > nlvc, arrange for
the first nlvb variables to be the nonlinear variables common to both
constraints and objectives (and the next nlvc - nlvb to be just in
constraints).  Adjust tables in "Hooking Your Solver to AMPL" to
describe the new ordering of nonlinear variables; Postscript for
this revised report is available by E-mail: ask
netlib@netlib.bell-labs.com to

	send 93-10 from research/nam

19930805:
  New option presolve_inteps (default 1e-6) gives tolerance for
rounding updated bounds on integer variables to integer values
during presolve:  if x.dlb and x.dub denote the new deduced lower
and upper bounds on x, then for $presolve_inteps < 1,
x.dlb :=  ceil(x.dlb - $presolve_inteps)  and
x.dub := floor(x.dub + $presolve_inteps).  For $presolve_inteps >= 1,
x.dlb := floor(x.dlb) and x.dub := ceil(x.dub).
  Adjust presolve complaints about impossible deduced bounds to
take $presolve_eps into account.

19930914:
  New dot notation variable.dual applies to defined variables and
gives the dual value for the defining equality constraint; this
gives the partial derivative of the Lagrangian function (objective
minus sum of duals times constraints) with respect to the defined
variable.  For other variables, variable.dual = 0.


19930928:
---------------
FLOW OF CONTROL
---------------
  Several new commands permit conditional execution of and
looping over lists of AMPL commands:

	if lexpr then cmd
	if lexpr then cmd else cmd	# else matches nearest
					# available if
	for opt_name indexing cmd	# dummies from indexing
					# may appear in cmd
	repeat opt_name opt_while { cmds } opt_while ;
	break opt_name ;
	continue opt_name ;

cmd is either a single command (ending with ;) or { cmds } .
cmds is a sequence of 0 or more commands .
lexpr is a logical expression.
opt_name is an optional loop name (which must be an unbound
before the syntactic start of the loop), which goes out of
scope after syntactic end of the loop.
opt_while clauses are optional.  If not null, opt_while
has the form

	while lexpr
or
	until lexpr

If the optional loop name is not specified, break and continue
apply to the immediately enclosing loop; otherwise they apply
to the named loop; break terminates the loop, and continue
causes its next iteration to begin (if permitted by the
optional initial and final opt_while clauses of a repeat loop,
or by the indexing of a for loop).

Loops and if-then-else structures are treasured up until syntactically
complete.  Because else clauses are optional, AMPL must look ahead one
token to check for their presence.  At the outermost level, one must
thus issue a null command (just a semicolon) or some other command or
declaration to execute an outermost else-less "if ...  then stmt".
(In this regard, end-of-file implies an implicit null statement.)

New options cmdprompt1 and cmdprompt2 (called prompt3 and prompt4
until 19930423) control prompting within the new flow-of-control
commands.

19931005:
  New options $abs_boundtol, $rel_boundtol, and $show_boundtol are
meant to help deduce correct dual values for constraints eliminated
by presolve when the solver uses an interior-point algorithm and
returns a solution with no bounds strictly holding.  All three new
options have default value 0, which gives the previous behavior.
Suppose for some variable x that the solver sees the bounds
lb <= x <= ub.  The lower-bound constraint lb <= x is considered
active (during reconstruction of dual values) if

	x <= lb
	or (x - lb < ub - x
		and x - lb <= max($abs_boundtol, |lb|*$rel_boundtol)),

and similarly for the simple upper-bound constraint (x <= ub).
Thus negative values of $abs_boundtol and $rel_boundtol behave
like 0.  The condition x - lb < ub - x ensures that x is closer
to lb than half-way between lb and ub, ensuring that AMPL picks the
more appropriate bound no matter how large $abs_boundtol and
$rel_boundtol are.
  New option $show_boundtol works similarly to $show_stats, except
that it delivers its messages when it is on (nonzero) and
another dual-value computation occurs or (like $show_stats)
when it is set to 1.  It reports changes to $abs_boundtol and
$rel_boundtol that would change the outcome of the dual computation,
and is silent if the values of $abs_boundtol and $rel_boundtol do
not matter.  [$show_boundtol was called $show_boundstats until
20 Dec. 1993.]
  Have option redirections affect printing for "option show_stats 1;"
and "option show_boundtol 1;".

19931029:
  Arrange for expressions involving primal and dual variable values
to be recomputed when those values change.  Example:
	set S := 1..6;
	var x;
	for{i in 2..4} {
		let x := i;
		display {j in S: j > x.val};
		}
formerly displayed the same set thrice; now each is distinct.
  Treat variable as variable.val, constraint as constraint.dual
in indexing expressions for "fix", "unfix", "drop", "restore".
  New syntax for fix and unfix commands: an optional := expr
may appear before the terminating semicolon, in which case the
expression is assigned to the variable being fixed or unfixed
(as though assigned by "let").
  New option ampl_include gives a white-space separated list of
directories in which to search for files in "include", "model",
and "data" commands.  In this list, a single period stands
for the current directory.  The default, '.' (a single period)
gives the same behavior as heretofore.  References to absolute
file names (starting with "/" or, for MSDOS, one of "/", "\", or
"x:", where x is any printing character) are not affected by
$ampl_include .

19931113:
  Add "exit" as synonym for "quit".
  Recognize file names that start with "./" as file names relative to
the current directory (and ignore $ampl_include for such file names).
  Arrange for all (?) expressions involving dot notation to be
recomputed when the "dot value" changes.  This involves adding
"system" parameters _Solution_Count and _Initial_Guess_Count that,
for debugging, may be referenced as params (but which may become
invisible later).
  New option presolve_intepsmax (default 1e-5).  The message
"Setting $presolve_inteps >= nnn might help" is suppressed if
nnn > $presolve_intepsmax.
  New option presolve_warnings (default 5) limits the number of warning
messages printed during presolve; subsequent warning messages are
suppressed, and the number of suppressed warnings (if positive) is
reported at the end of presolve.  When $presolve_warnings < |$eexit|
(as is true by default), a subsequent "Ignoring solve command
because presolve finds no feasible solution possible" may now appear
even when presolve finds at least |$eexit| causes for infeasibility.

19931123:
  New option log_file (default '').  If $log_file is not '', then all
lines read from stdin or written to stdout or stderr are copied to file
$log_file.

19931203:
  New option bad_subscripts: ampl now discards invalid subscripts
(read in a data section or assigned by "let"), and the accompanying
error message now shows at most $bad_subscripts (default 3) invalid
subscripts per entity (when there is more than one bad subscript).

19940303:
  Allow any option value that does not need to be quoted in a
data section to be unquoted in option commands.  Option values
have always been printed without quotes when quotes can be elided
in a data section, which made it impossible for $OPTIONS_INOUT to
restore a value like a.b (or just ".", the default value for
$ampl_include).  Side effect: numeric option values are no
longer rounded (to the shortest decimal string rounding to their
numerical value rounded to the machine's arithmetic).  For example,
previously

	option foo 00123, goo '00123', zoo 1.234567890123456789;
	option foo, goo, zoo;

printed

	option foo 123;
	option goo 00123;
	option zoo 1.2345678901234567;

and now it prints

	option foo 00123;
	option goo 00123;
	option zoo 1.234567890123456789;

19940317:
  Allow "let S := {};" for sets S of arbitrary arity.

19930423:
  Allow any UTF character beyond the 7-bit ASCII characters to appear
in names.
  New options dataprompt1 and dataprompt2 are analogous to prompt1
and prompt2, but for data mode; defaults 'ampl data:' and 'ampl data?'.
  Catch SIGINT ("break" or "del" key).  When received, terminate
reading all files except stdin, and abort compound commands.
Stop if a second SIGINT arrives before a successful read on stdin.

19940429:
  Abort compound commands when solve returns a nonzero status
(e.g., if the solver was stopped by a SIGINT).

19940506:
  Test whether variables fixed by the "fix" command lie within
$presolve_eps of their declared ranges.

19940512:
  Arrange that
	set A default {expr};
	...
	let A := A;
will keep A at its current value when the value of expr changes.  Before
A is assigned a value by let (or in a data section), the value of A will
change when expr changes value.

19940807:
  Abort compound commands when solve or write says "Ignoring ...".
  Rerun presolve after changes to $presolve_eps.
  Allow inconsistencies up to $presolve_eps in declared variable and
constraint lower and upper bounds.
  For inconsistent problems (detected by presolve), tell changes to
$presolve_eps that would make AMPL ignore the inconsistencies,
provided the larger $presolve_eps would be at most $presolve_epsmax
(a new option with default 0).  Report changes (below $presolve_epsmax)
to $presolve_eps that would affect presolve results with $show_stats
output.
  New options presolve_fixeps and presolve_fixepsmax (both with
default 0): if presolve finds or deduces lower and upper bounds on
a variable that differ by at most $presolve_fixeps, it fixes the
variable at the average of the bound values.  When changes below
$presolve_fixepsmax to $presolve_fixeps would affect presolve,
the $show_stats output reports these changes.  Presolve now behaves
as though $presolve_eps were max($presolve_eps, $presolve_fixeps):
when $presolve_eps < $presolve_fixeps, variable bounds declared or
deduced to be within $presolve_fixeps of each other in absolute
value result in the variable being fixed at the average of the bounds.

19941003:
  Instantiate the entire index set of a for loop before starting
execution of the loop, so the set of dummy indices for which the loop
body is executed will be unaffected by assignments in the loop body.
Example:
	set S default {1,2,3,4};
	for {i in S} let S := S union {i + 4};
	display S;	# used to give 1..5; now gives 1..8
	## The loop could be stated more efficiently:
	##	let S := S union setof{i in S} i+4;
  Allow write and solve commands to proceed if only error messages
about discarded subscripts appear.
  Disallow write and solve commands when presolve complains about
inconsistent bounds; at the second attempt, show the least value
of $presolve_eps that would allow the command to proceed.
  Apply $presolve_fixeps test to the declared bounds on each
variable; although the description of $presolve_fixeps suggested
that it should apply to the declared bounds, it did not.
  Change to behavior of "model filename" and "data filename", which
are now commands: AMPL returns to model mode at the end of the file
unless the file ends in the middle of data.
  Change to behavior of "data" and (hitherto undocumented) "commands"
commands: when they appear within a compound command (i.e.,
the body of a loop or the then or else part of an if command,
or simply in a sequence of commands enclosed in braces),
they are now executed when the flow of control reaches them,
instead of when the compound command is being read.  In this case,
if they do not specify a file, AMPL reads commands or data from the
current input file until it encounters either an "end" command or
the end of the current file.  New option insertprompt (default '<%d>'),
if nonnull, specifies an insert-prompt (in which %d is the current
insert level, i.e., nesting of "data" and "commands" commands
specifying files and appearing within a compound command) that
immediately precedes the usual prompt for input from stdin.
  New single-step mode, details of which may change:

	option single_step n;

where n is a positive integer, specifies that if the insert level
is at most n, AMPL should behave as though "commands -;" were inserted
before each command: it should read commands from stdin until "end" or
eof (control-D on Unix systems).  Some special commands may appear in
this mode:

	command		meaning

	step		execute the next command

	skip		skip the next command

	next		if the next command is an if-then-else
			or looping command, execute the entire
			compound command before stopping again
			(unless the compound command itself
			specifies "commands -;")

	cont		execute until the end of the end of all
			currently nested compound commands at the
			current insert level

  Allow "reset data" and "update data" commands to appear in
compound commands.
  New option solver_msg (default 0; called omit_solmsg until 19941007):
if set to 0, the solution message normally printed by the solve and
solution commands is suppressed.

19941028:
  Omit (for now) recognition of @ (which once upon a time was a
synonym for "cross" but now is reserved for future use).
  Adjust some insert-mode details; commands read by "include" or "model"
are now at insert-level 0.
  New "read" command with syntax similar to the print command, except
that the only simple or subscripted params, variables, and constraints
(dual values) can be read.  Optional input redirections are specified
by < filename or < 'quoted_file_name' (before the read command's
terminating semicolon).  If no redirection is specified, values are read
from the current input stream.  To read from stdin, specify <- .
Examples (reading from the current input steam):

	param p;
	read p;
	4.2
	display p;

	param ps symbolic;
	read ps;
	'some funny text\
	with a newline'
	display ps;

	param q{1..3};
	read{i in 1..3} q[i];
	1.1 2.2
	3.3 display q;

	param i;
	param j;
	param A{1..10,1..10};
	param n;
	read n,{1..n}(i,j,A[i,j]);
	3
	2 4 2.4
	3 5 3.5
	4 7 4.7
	display A;

19941127:
  Omit check for violation of declared bound constraints by variables
fixed with the "fix" command.
  Abort compound commands (and the reading of files other than stdin)
after "Ignoring solve command..." or "Ignoring write command...".

19950315:
  Provisional versions (subject to change as we gain experience and
get feedback) of several extensions are now available.

  New reserved word "all", used in "drop all;", "fix all;",
"restore all;", "unfix all;".

  Extension to "objective" command: "objective;" or
"objective >filename;" prints commands establishing the current drop
status of objectives.  In particular, if one had previously said
"objective foo[3];", "objective;" would print "objective foo[3];".

  Similarly, "drop;" or "drop >filename;" or "restore;" or
"restore > filename;" prints commands establishing the
current drop state of the constraints and objectives, and
"fix;" or "fix >filename;" or "unfix;" or "unfix >filename;"
prints commands establishing the current "fix" state
of the variables.  In these contexts, "drop" and "restore"
are interchangeable, as are "fix" and "unfix".

  New "problem" declaration/command has three functions:
declaring a new problem, making a previously declared problem
current, and printing the name of the current problem (in the form
of a problem command establishing the current problem).

	problem name optional_indexing optional_environ : itemlist ;

declares a new problem and specifies the variables, constraints,
and objectives that are in it.  Other variables appearing in the
specified constraints and objectives are fixed (but can be
unfixed by the "unfix" command).  The new problem becomes the
current problem.  Initially the current problem is "Initial".
The "itemlist" in a problem declaration is a comma-separated
list of possibly subscripted names of variables, constraints,
and objectives, each optionally preceded by an indexing, as in
{i in A} foo[i].  More generally, nested indexings similar to
those allowed in function calls may be specified, as in
	{i in A} (foo[i], goo[i], {(i,j) in B} zoo[i,j])

The command

	problem name;

makes name (a previously declared problem) current.  And

	problem;
or
	problem >filename;

prints the current problem name (as "problem name;").
Drop/restore and fix/unfix commands apply only to the
current problem.  Variable values, like params, are global;
just the fixed/unfixed status of a variable depends on the
problem.  Similarly, the drop/restore status of a constraint
depends on the problem (as do reduced costs).  The
current problem does not restrict the "let" command.

When a problem is declared, it can optionally specify an
environment associated with the problem: the optional_environ
phrase has the form

	environ envname

to specify that the problem's initial environment is envname.
Otherwise a new environment with the same name as the problem
is created, and it inherits the then current environment (set of
option values).  In option commands, unadorned (conventional)
option names refer to options in the current environment, and
the notation envname.opname refers to $opname in environment envname.
The new declaration

	environ envname optional_indexing;

declares a environment envname (or a set of environments,
subscripted by the indexing if specified).  If there is no indexing,
envname becomes the current environment for the current problem.

  New command

	environ optional_indexing envname := envname1;

where envname and envname1 are optionally subscripted environment names,
copies environment envname1 to envname.

  New "expand" command prints generated constraints and objectives
(much as the linrc program does):

	expand [indexing] itemlist [>filename];

The itemlist can assume the same forms allowed in problem declarations.
If it is empty, all non-dropped constraints and objectives are expanded.
The variant

	solexpand [indexing] itemlist [>filename];

shows how constraints and objectives appear to the solver.  It omits
constraints and variables eliminated by presolve unless they are
explicitly specified in the itemlist.

  Both the "expand" and "solexpand" commands permit variables to appear
in the itemlist; for each, the commands show the linear coefficients
of the variable in the relevant (non-dropped and, for solexpand,
not eliminated by presolve) constraints and objectives, and indicates
" + nonlinear" when the variable also participates nonlinearly in a
constraint or objective.

  New options expand_precision and expand_round control printing of
numbers by expand.  By default they are currently printed to 6
significant figures (option expand_precision 6).

  Change to function calling conventions: symbolic arguments were
formerly quoted (as though they were symbols in a data section);
now they are stripped of quotes and the \ before a newline.  Examples:

	print 'a b';

now prints

	a b

rather than

	'a b'

  New printf format %q prints with data-section quoting rules
(omit quotes if omitting them is allowed in a data section);
new printf format %Q always quotes strings.

  New concatenation operator & has precedence below all arithmetic
operators and performs string concatenation.  It accepts numeric
operands and converts them to full-precision decimal strings
(as though by printf format "%.g": recall that AMPL's "%.0g"
gives full precision, rather than behaving like "%.1g").

  Contexts (other than alias strings in declarations) that previously
required literal strings now also accept an expression in parentheses.

  Expressions in commands may involve operands of the form $value
(a $ followed by an environment name) and $environ.value (where
environ is the possibly subscripted and previously declared name of
an environment).  $values may not be used in declarations.

  New builtin functions involved with strings:

	num('12.34') = 12.34	# convert string to number
	num('12.34x') = error	# complain if stripping leading and
				# trailing white space doesn't yield
				# a valid decimal number
	num0('12.34x') = 12.34	# strip leading white space, and
				# interpret as much as possible as
				# a number, but never complain
	ichar('A') = 65		# Unicode value of the first character
				# in its argument string
	char(65) = 'A'		# inverse of ichar
	length('abcd') = 4	# length of string
	substr('abcdef',3,2) = 'cd'	# substring
	substr('abcdef',3) = 'cdef'	# substring
	sprintf("stuff %.2f blah %g Blah %.g", 13/3, 2/7, 3/11)
		= 'stuff 4.33 blah 0.285714 Blah 0.2727272727272727'
				# general formatted conversion to string
	match('abcde','cd') = 3 # starting position of arg2 in arg1
	match('abcde','xy') = 0	# or 0 if not found; arg2 can be a general
				# regular expression
	sub('abcdecd','cd','XYZ') = 'abXYZecd'
				# substitute arg3 for the first occurrence
				# of arg2 in arg1
	gsub('abcdecd','cd','XYZ') = 'abXYZeXYZ'
				# substitute arg3 for all occurrences of
				# of arg2 in arg1
	arity('S') = arity of S if S is a set; else 0
				# for use with _SETS, as in
				# display{s in _SETS} arith(s);

  There is no implicit conversion of strings to numbers, but new
builtin functions num(string) and num0(string) perform explicit
conversions.  Both ignore leading and trailing white space; num
complains if what remains is not a valid number, whereas num0 just
converts as much as it can (returning 0 if it sees no digits).

The expressions
		'abc' & x+3
and
		'abc' & sprintf("%.g",x+3)

yield the same strings.  Now, e.g.,

	setof{i in 1..3} 'ABC' & i = {'ABC1', 'ABC2', 'ABC3'}.

The match, sub, and gsub functions accept strings representing
regular expressions as their second arguments.  Such expressions
are as in plan 9.  They are similar to the regular expressions
recognized by the Unix editors ed and sed, except that parentheses
as operators must not be escaped, and, in addition to * for 0 or
more occurrences of the preceding item, + means 1 or more occurrences,
and ? means 0 or 1 occurrence.  The replacement patterns (third
arguments) for sub and gsub are like those for ed and sed: & stands
for the whole matched string, as does \0, and \1, \2, ... \9 stand
for the string matched by the first, second, ..., ninth parenthesized
expression in the pattern.

  New "cd" command reports or changes the current working directory.

  New automatically updated params

	_nvars = number of variables in current model
	_ncons = number of constraints in  "	  "
	_nobjs = number of objectives in   "	  "
	_varname{1.._nvars} = names of variables in current model
	_conname{1.._ncons} =   "   "	    "	   "	"	"
	_objname{1.._nobjs} =   "   "	    "	   "	"	"

and synonyms for current model entities:

	_var{1.._nvars} = synonyms for variables   in current model
	_con{1.._ncons} =     "	    "  constraints  "	"	"
	_obj{1.._nobjs} =     "	    "  objectives   "	"	"

These synonyms can be used in display and other commands.  They
present the modeler's view (before presolve).  Similarly automatically
updated entities with _ changed to _s (i.e., _snvars, _svarnames,
_svar, etc.) give the solver's view, i.e., the view after presolve.

  New automatically updated sets:

	_PARS	= set of all declared param names
	_SETS	=  "   "   "	"     set      "
	_VARS	=  "   "   "	"     variable "
	_CONS	=  "   "   "	"     constraint names
	_OBJS	=  "   "   "	"     objective    "
	_PROBS	=  "   "   "	"     problem	   "
	_ENVS	=  "   "   "	"     environment  "
	_FUNCS	=  "   "   "	"     (user-defined) functions

These sets may appear in commands, such as display and print.

  New additions to the reserved-word list: Current, Initial, all,
environ, option.  The other new names appearing above may be declared
to be something else.

19950412:
  Enforce the restriction that $values and synonyms (such as _VARS
and _nvars) may not appear in declarations.
  Abort reading commands from files other than stdin after
detecting an error.
  WATCOM binaries are now compiled to circumvent the Pentium divide bug.
  Preliminary implementations of xref, delete, purge, and redeclare:
  New command xref shows entities that depend directly or indirectly
on specified entities.  Syntax:
	xref itemlist optional_redirection;
  New command
	delete foo;
deletes foo, restoring any previous meaning foo had, provided no other
entities depend on foo, i.e., if "xref foo;" reports no dependents.
  New command
	purge foo;
deletes foo and all its (direct or indirect) dependents.
  New form of declaration: "redeclare" followed by an ordinary
redeclaration replaces any existing declaration of the specified
entity with the given one, provided either that the entity has no
dependents, or that the new declaration does not change the character
of the entity (its kind, such as set or param, and its number of
subscripts).  Redeclarations that would cause circular dependencies
are rejected.
  New symbolic system parameter solve_message is assigned the message
shown (when not suppressed by "option solver_msg 0;") by the solve
and solution commands.  One can assign solve_message with "let", but
may not delete or redeclare it.
  New variants of break:
	break commands;
	break all;
terminate, respectively, the current commands command or all commands
commands, if any, and otherwise act as a "quit" command.

19950416:
  Extend $solver_auxfiles interpretation: capital letters have no
affect on linear problems, but on nonlinear problems (including
problems with binary or integer variables) are treated the same as
their lower-case equivalents.  The default $minos_auxfiles is now RF
rather than rf, causing .row and .fix files to be written only for
nonlinear problems.

19950501:
  New function
	indexarity('foo')
gives the arity of foo's index set if foo has been declared to be
something indexed, or 0 if foo has been declared as something that
is not indexed, or -1 if foo has not been declared.  Example:
	display{i in _PARS} indexarity(i);

19950517:
  Check statements are now only enforced during solve, write, and
solution commands, or when the new command
	check;
is executed.
  Dot notations that require presolve are now excluded from
declarations.

19950521:
  New variant of close command:
	close;
closes all files opened by redirections.

19950614
  Arrange for option funcwarn only to affect constraint and objective
declarations.  Thus it no longer affects
	function zot;
	param p := zot(3); # used to get msg about variable in :=
	display p;	   # now elicits msg about zot missing
  Change in writing of auxiliary .fix file: values of variables
fixed by presolve are suppressed unless $auxfiles or
$($solver & '_auxfiles') contains "v".  (The primary reason use
of the .fix file is now to convey names of defined variables
for use in printing error messages.  This change omits sometimes
lengthy work that is hardly ever useful.)
  Warn of missing subscripts on sets.  (Later we hope to warn about
other missing subscripts, but that's harder.)

19950619
  Introduce variant _display of the display command; _display is meant
for possible use by front ends.  Like the display command, _display
emits one or more tables, but _display replaces display's table
header with a line consisting of "_display" and three integers:
	s = number of subscripts of each item in each table line
	k = number of items on each table line
	n = number of table lines
Each table line consists of s subscripts followed by k items,
all separated by commas.  No semicolon is appended to the table.

19950720
  Add another variant of the display command: csvdisplay is similar to
_display, except that it only writes one table (complaining if asked
to write more than one), and it replaces the initial _display output
that describes the following table with $1,$2,...,$n  (where n is the
number of items in one line of the table that follows).  New options
csvdisplay_precision and csvdisplay_round govern the precision of
printing for csvdisplay; the defaults  (0 and '') give full precision.

19950726
  Adjust _display's printing so numeric precision is governed by
$csvdisplay_precision and $csvdisplay_round.
  In -M output and printing by the show command, insert "s.t. "
before constraints.
  New option csvdisplay_header (default 1): if nonzero, cvsdisplay's
first line now has the form
	Index_1,Index_2,...,Index_k,Expr_1,...,Expr_n
(where k is the number of subscripts and n the number of
expressions in one line of the table that follows); if the jth
expression is a simple name (not of the form Index_i or Expr_i)
that has not appeared previously in the current csvdisplay header
line, then that name appears rather than "Expr_j".  If
$csvdisplay_header is 0, this header line is omitted.
  Adjust line breaking for "show" and omit some extraneous commas.
  Diagnose some more undefined names (e.g., in problem declarations).
  Recognize repetition counts in
	next nnn
	skip nnn
	step nnn

19950810
  Allow the print command to have no arguments, which causes
it to behave the same as
	printf "\n";

19950827
  New debugging option: $debug_stub, if not '' (its default value),
is used to construct stubs of the form ($debug_stub & '_1'),
($debug_stub & '_2'), ... in solve commands.  If a file named
($debug_stub & '_1.sol'), ($debug_stub & '_2.sol'), etc. exists,
"solve" behaves as "solution" and reads the .sol file.  Otherwise it
leaves behind its .nl and .sol files, so a subsequent ampl run can
read the .sol files (and the .nl files are available for debugging
purposes).  This permits recreating some situations without rerunning
the solvers involved.  The .nl files (and hence .sol) files are ASCII
files to make it easy to create them on one kind of machine and use
them on another.

19951009
  New command:
	reset function;
closes all pipe functions and unlinks all dynamically linked functions,
causing them to be restarted or relinked if invoked again.  For
a specific function foo,
	reset data foo;
acts just on function foo.
  New command:
	delete check n;
deletes the n-th check.
  Extension of redeclare syntax:
	redeclare check n optional_indexing : ...;
redeclares the n-th check.
  New addition to reserved-word list: dotname (for use in a forthcoming
extension).

19951020
  Allow "reset data p;" even when p is declared with a := expression,
to force recomputation of random functions in the := expression (and
to force reevaluation of any user-defined functions in the expression).
Extend "reset data;" to force recomputation of all := expressions.

19951027
  Now problems (including the current one) should be adjusted when
their indexing expressions change, except that previous explicit drop,
restore, fix, and unfix commands remain in effect.  The new
"reset problem" command cancels this treatment of previous explicit
drop, restore, fix, and unfix commands, and brings the problem to its
declared drop/fix state.  This command has the forms

	reset problem;		# applies to the current problem
	reset problem foo;
	reset problem goo[subscripting];

If the latter forms mention the current problem, they have the same
effect as the first form.  For now, at least, "reset problem"
does not affect the problem's environment.

19951202
  Arrange for
	option relax_integrality 2;
to assume integrality during presolve, but tell the solver that there
are no integer variables.  Force presolve to run again after
$relax_integrality has changed.
  Increase default value of $pl_bigM from 1e4 to 1e6.  Arrange for
changes to $pl_bigM to cause presolve to run again when its results
would be affected by the change to $pl_bigM.

19951204
  Arrange to run presolve (when nothing else has caused it to run) so
defined variables involved in dropped objectives or constraints appear
to have the "right" values.  Example:
	var x := 2;
	var y = x^2 + 1;
	minimize zot: y + 10;
	drop zot;
	print zot;	# used to print 11 rather than 15
  Disallow declarations involving defined_variable.val.  (Instead,
declare a new param, use the param in the declaration, and use "let"
to give it the right value.  This helps clarify when things are to be
evaluated.)

19960202
  Adjust DOS .exe files to print nonzero return codes from solve
and shell commands.

19960306
  Allow printf's "+" flag to reveal the sign of 0.

19960313
  Arrange for redeclarations of variables, constraints, and objectives
to retain current values and drop/fix status.

19960330
  New call command for directly invoking user-defined functions
for their side effects: rather than, e.g., saying "print foo(1,2,3);"
or "let Dummy := foo(1,2,3);" to force foo(1,2,3) to be called, you
can now say "call foo(1,2,3);".  Syntax:
	call [indexing] function [(arglist)];

19960418
  Terminate includes within the file in a read command's "<" redirection
when the read command ends.

19960425
  Adjust xref command so it does not show nodes cited in arc
declarations.  Make it possible to delete and purge arcs.

19960606
  Adjust option debug_stub to honor (and retain) ($solver)_auxfiles.
  Pretend a semicolon appears at the end of command files that end with
a compound command with optional final parts missing:
	repeat ...  { ... }	# no final condition or semicolon
	if ... then { ... } 	# no else clause

19960627
  New option (for debugging nonlinear models): option nl_permute
(default 3) tells whether to permute constraints and variables as
described in "Hooking Your Solver to AMPL".  The value, mod 4, tells
what to permute:
		0	nothing
		1	just constraints
		2	just variables
		3	both constraints and variables

Note that some solvers, such as cplex, minos, and osl, require the
permutations.

19960709
  Prevent execution of problem commands from running presolve (and thus
possibly giving an erroneous message about inconsistent constraints).

19960729
  Disallow writing MPS files for problems with nonlinear variables.
  Omit INITIAL bounds from .mps files; they were only needed long ago
by minos.
  New builtin functions: time() returns the current time (in seconds
since 00:00:00 1 Jan. 1970 GMT); ctime() returns a 24-character string
representing the current time, and ctime(t) returns a similar time
representing time t (or the current time, if t is not numeric).

19960816
  New predefined param _pid gives the process ID of the AMPL process
(a number unique among processes running on the system).
  New option format_range_errmsg, if not '', replaces %i, %d, %o, %u,
%x, or %X in printf commands or sprintf invocations in which the
number to be converted is outside the appropriate range of integers.
If $format_range_errmsg has its default value of '', an error message
appears and the command is aborted.
  New automatically determined set:
	_AVAILFUNCS = user-defined functions that have been linked
(statically or dynamically) with AMPL and thus can be declared and
evaluated in the AMPL session.  (Other user-defined functions may be
declared and used in constraints and objectives, but AMPL will not
be able to evaluate them.  It can, however, pass them by name to
solvers that know how to evaluate them.)
  New automatically updated params give some problem statistics
the solver sees:
	_snbvars  = number of binary variables used linearly
	_snivars  = number of general integer variables used linearly
	_snnlcons = number of nonlinear constraints
	_snnlobjs = number of nonlinear objectives
	_snzcons  = number of nonzeros in the constraint Jacobian matrix
	_snzobjs  = number of nonzeros in objective gradients

19960829
  Add "inout" and "out" to the reserved-word list (for use in upcoming
extensions).

19961017
  New option show_write_files controls whether the write command
reports the names of the files it writes:
	0 ==> no (default)
	1 ==> yes for "write;", no for "write gstub;"
	2 ==> yes

19961210
  Adjust most commands (but not declarations) to permit unqualified
objective, constraint, and variable names in () expressions.  Hitherto
one had to use objective.val, constraint.dual, and variable.val.  Now
one can write something like
	option cplex_options (sprintf('... uppercutoff=%.f ',_obj[1]));
(rather than _obj[1].val) to adjust an option to reflect an objective
value from the most recent solve.
  Disallow expressions involving variables in arguments to member, ord,
ord0, next, nextw, prev, and prevw, when they appear in constraint and
objective declarations.

19970307
  Arrange for variable values of 0 assigned by "let" to be explicitly
conveyed to solvers unless $reset_initial_guesses is nonzero.
Previously they were explicitly conveyed only after a solve, or if
they were given in a data section or variable declaration.

19970314
  Arrange for "option gentimes 2" to show total memory rather than
incremental memory.
  Tweak -T output so "genmod times" lines (other than headers) always
start with "## ".  This was hitherto only true for seq numbers <= 99.

19970528
  Extension to expand command: recognize synonyms (possibly subscripted
_con, _obj, _var, _scon, _sobj, _svar).

19970615
  Adjust the tests involving $pl_bigM (done when linearizing nonconvex
piecewise-linear terms) to assume a lower bound of
		(smallest breakpoint) - $pl_bigM
and upper bound of
		(biggest breakpoint)  + $pl_bigM
on the variable "multiplied" by a piecewise-linear coefficient when
the true bounds on the variable exceed these values (instead of just
when the true bounds are infinite).

19970729
  Fix bug with random functions that caused some expressions involving
them not to be reevaluated.  Example:
	set S default {};
	repeat { let S := S union {Uniform(0,15)}; } while(card(S)) < 5;
looped infinitely.
  Change exit to a command of the form
	exit ;
or
	exit expression ;
The expression, if given, must be numeric and specifies the process
return code; an omitted expression is treated as 0.
  Change "quit;" to be equivalent to "exit <oldrc>;", which means it may
now appear in compound commands.  Here, <oldrc> stands for the return
code that "quit" has always given, which reflects recent syntax errors
(and is cleared to 0 by successful command executions).
  Fix some bugs with "break commands;" and "break all;"; adjust these
commands so if no "commands" command is being executed, they terminate
the current nest of includes, if any, and otherwise are no-ops.

19970731
  Fix bug in presolve for problems with integer (or binary) variables:
for $presolve > 1, if a constraint with two remaining nonzeros caused
an integer variable to be fixed and the constraint had previously
implied a bound on the other variable, that bound was not conveyed
to the solver.  (Note that $presolve is 10 by default; specifying
"option presolve 1;" avoids the bug.)
  Add a Caution for writing an MPS file for problems with integer
variables having infinite bounds.  There's no "standard" MPS way to
express such problems.

19970814
  With invocations with -o (and no commands that would cause the -o
command-line option to be ignored), if presolve fixes all variables,
say so and give process return code 16 rather than 0.
  New command-line option -R (recognized only as the first command-line
option and not mentioned in the "-?" listing of options) puts AMPL into
"server mode", in which it declines to execute cd and shell commands,
forbids changes to options TMPDIR, ampl_include, and PATH (or whatever
is the usual name of the search path for the operating system being
used), disallows pipe functions, and restricts names in option solver
and file redirections to be alphanumeric (so they can only write to the
current directory, which, on Unix systems at least, cannot be changed).
By invoking ampl from a shell script that suitably adjusts the current
directory and environment variables TMPDIR, ampl_include, and PATH
before it invokes "ampl -R", one can control the environment in which
"ampl -R" operates.

19980127
  New syntax for expressing complementarity constraints: in addition
to previous forms, constraint declarations may now have the form
	constraint_start : constr1 complements constr2 ;
in which constr1 and constr2 consist of 1, 2, or 3 expressions
separated by the operators <=, >=, or == (or =).  As with other
constraint declarations, constraint_start gives a name to the
constraint, an optional descriptive string (alias), and, if the
declaration describes a collection of constraints, an indexing
expression.  In constr1 and constr2 together, there must be a total
of two explicit inequality operators, with == counting as two.  A
complementarity constraint is satisfied if both constr1 and constr2
hold and at least one inequality is tight, i.e., satisfied as an
equality.  If one of constr1 or constr2 involves two inequalities,
then the constraint must have one of the forms
	expr1 <= expr2 <= expr3 complements expr4
	expr3 >= expr2 >= expr1 complements expr4
	expr4 complements expr1 <= expr2 <= expr3
	expr4 complements expr3 >= expr2 >= expr1
In all of these cases, the constraint requires the inequalities to
hold, with
	expr4 >= 0 if expr1 == expr2
	expr4 <= 0 if expr2 == expr3
	expr4 == 0 if expr1 < expr2 < expr3
  For expressing MPECs (math. programs with equilibrium constraints),
complementarity constraints may coexist with other constraints and
objectives.
  The suffix notations, such as constraint.lb, constraint.body, etc.,
are extended so that for a complementarity constraint,
constraint.Lsuffix and constraint.Rsuffix correspond to constr1.suffix
and constr2.suffix, respectively, and complementarity_constraint.slack
(or the unadorned name) stands for a measure of the extent to which
the complementarity constraint is satisfied: if constr1 and constr2
each involves one inequality, then the new measure is min(constr1.slack,
constr2.slack) (which is positive if both are strictly satisfied,
0 if the complementarity constraint is satisfied exactly, and negative
if at least one of constr1 or constr2 is violated).  For constraints of
the form expr1 <= expr2 <= expr3 complements expr4, the .slack value is
	min(expr1-expr2,  expr4) if expr1 >= expr2
	min(expr2-expr3, -expr4) if expr3 <= expr2
	-abs(expr4)		 if expr1 < expr2 < expr3
so in all cases, the .slack value is 0 if the complementarity constraint
holds exactly and is negative if one of the requisite inequalities
is violated.
  Solvers see complementarity constraints in the standard form
	expr complements lb <= variable <= ub
A new synonym, _scvar{i in 1.._sncons}, indicates which variable, if
any, complements constraint _scon[i]: if _scvar[i] in 1.._snvars, then
variable _svar[_scvar[i]] complements constraint _scon[i]; otherwise
_scvar[i] == 0, and _con[i] is an ordinary constraint.  Other new
synonyms: _cconname{1.._nccons} are the names of the complementarity
constraints as the modeler sees them.
  A forthcoming paper ("Expressing Complementarity Problems in an
Algebraic Modeling Language and Communicating Them to Solvers", by
Michael C. Ferris, Robert Fourer, and David M. Gay) discusses the new
complementarity syntax.  The paper will be available from the AMPL web
site (http://www.ampl.com/ampl/) after it has gone through the Bell Labs
release process.
  Anticipating extensions to be described later, the following words
	IN
	INOUT
	LOCAL
	OUT
	suffix
are now reserved.  This breaks the blend.mod and blend.dat distributed
with the AMPL book, in which it's necessary to change OUT to Out and
IN to In.

19980128
  Introduce some new automatically updated params that give information
about the current problem, either as seen by the modeler (_nccons), or by
the solver (names that start with "_s"):
	Name		Meaning
	_nccons		No. of complementarity constraints before presolve
	_snbvars	No. of binary (0,1) variables
	_snccons	No. of complementarity constraints after presolve
	_snivars	No. of  general integer variables (excluding binaries)
	_snlcc		No. of linear complementarity constraints
	_snlnc		No. of linear network constraints
	_snnlcc		No. of nonlinear compl. constrs.:
				_snccons = _snlcc + _snnlcc
	_snnlcons	No. of nonlinear constraints
	_snnlnc		No. of nonlinear network constraints
	_snnlobjs	No. of nonlinear objectives
	_snnlv		No. of nonlinear variables
	_snzcons	No. of constraint Jacobian matrix nonzeros
	_snzobjs	No. of objective gradient nonzeros

19980208
  New builtin suffixes: variable.defeqn is the subscript of _con for
the corresponding defining constraint if the variable is a defined
variable, and is 0 otherwise.  Similarly, constraint.defvar is the
subscript of _var for the resulting defined variable if the constraint
defines a defined variable (either through the var = syntax or because
of option substout), and is 0 otherwise.
  New options log_model and log_data (default 0): if option log_file
is not '', declarations and commands in included files are copied to
$log_file if $log_model is 1, and included data sections are copied
to $log_file if $log_data is 1.
  Edit change description of 19950619 above to describe _display.
For convenience, here is a summary of _display and csvdisplay.
  Commands _display and csvdisplay are variants of display that emit
tables in a more regular format than does display:  each line of a
table starts with s subscripts and ends with k items, all separated
by commas.  _display and csvdisplay differ in the table headers they
emit.  The header for _display consists of a line starting with
"_display" and followed by three integers s, k, and n (the number of
table lines that follow the header), each preceded by a space.  Whether
csvdisplay emits a header is determined by option csvdisplay_header
(default 1): if $csvdisplay_header is 1, cvsdisplay's first line has
the form
	Index_1,Index_2,...,Index_k,Expr_1,...,Expr_n
(where k is the number of subscripts and n the number of
expressions in one line of the table that follows); if the jth
expression is a simple name (not of the form Index_i or Expr_i)
that has not appeared previously in the current csvdisplay header
line, then that name appears rather than "Expr_j".  If
$csvdisplay_header is 0, this header line is omitted.
  Options csvdisplay_precision and csvdisplay_round govern the
precision of printing for _display and csvdisplay; the defaults
(0 and '') give full precision.

19980302
  Introduce (on all systems other than AIX and MSDOS, but including W95
and NT) a new scheme for acquiring imported functions: execute ampl -i?
for more details.  More extensive documentation will appear later.

19980309
  Change to option pl_linearize: the default value (1) now uses a
more efficient linearization of nonconvex (resp. nonconcave)
piecewise-linear terms and uses suffixes .sos and .sosref to
tell solvers that handle SOS sets about the variables and constraints
that imply the SOS2 constraints introduced by the linearization.
A forthcoming update to the AMPL/solver interface library will
provide sample interfaces that exploit this new information.
To get the old behavior, specify
	option pl_linearize 2;
Versions 19980305 and 19980308 had glitches in this change.

19980325
  New options
	shell_exitcode_max (default 2^16)
	solve_exitcode_max (default 0)
cause compound commands to be aborted if a shell command results in
$shell_exitcode > $shell_exitcode_max or a solve command results in
$solve_exitcode > $solve_exitcode_max .  The defaults give the old
behavior (except for the unlikely case of shell_exitcode > 2^16).

19980408
  Remove prohibition on "reset data" and "update data" commands
appearing in compound commands.

19980525
  When "solve;" fails, set solve_result = '?', solve_result_num = -1,
and solve_message = '?'.

19980605
  Recognize _obj.astatus, _sobj.statsus, _obj.astatus_num, and
_sobj.astatus_num.  The latter two are always 0 (in the problem), the
former two "in" unless $astatus_table has been changed from its default
value.

19980615
  Permit potential defining constraints (other than those introduced by
"var ... = ..." declarations) to participate in the strong bound
deductions attempted when $presolve > 1, unless $substout mod 4 == 2.
Previously, if there were any "var ... = ... " declarations or
$substout was nonzero, potential defining constraints were excluded
from the strong bound deductions.  Specifying "option substout 2" gives
the old behavior of "option substout 0", and "option substout 3" gives
the old behavior of "option substout 1".  The new $substout values 2 and
3 are for possible debugging and their effect may be withdrawn later;
values 0 and 1 are the only ones that should normally be of interest.
  Adjust logic for checking conditions on sets and parameters: when
"update data" and "let" provide new subscripts or modify existing
values, just check the new or changed items.  This changes the time
for some loops from quadratic in the number of iterations to linear.
If reports of failed conditions are interrupted by $eexit, the list
of questionable subscripts is currently not purged.  Otherwise each
new value causes only one warning if it fails a condition.

19980625
  New builtin param _cmdno is 0 initially (or after reset) and is
incremented each time a command is executed.
  New option project_fixed_vars (default 1): if nonzero, presolve
projects fixed variables onto their bounds (even if $presolve is 0).
If $project_fixed_vars == 2, a message is printed for each fixed
variable that is projected (until $presolve_warnings such messages
have appeared).
  It is possible for a variable to appear only in constraints that
imply bounds on the variable.  When $presolve > 0, such constraints
are eliminated.  Presolve now projects such variables onto the interval
given by their tightest deduced bounds.  Here is an example illustrating
this and the new option project_fixed_vars:
	var x >= 3 <= 3 := 4;
	var y >= 0 := 7;
	s.t. foo: x + y <= 3.5;
	display y;
	fix x;
	solexpand;
	display y;

Previously, this example gave the output

	y = 7

	presolve, variable y:
		impossible deduced bounds: lower = 0, upper = -0.5
	Infeasible constraints determined by presolve.
	s.t. foo:
		 <= -0.5;

	y = 7

The message about infeasibility appeared because "fix x" fixed x at 4,
which made constraint foo impossible to satisfy.  Now the example gives

	y = 7

	Solution determined by presolve.
	y = 0.5

Notice that y has been projected onto the interval [0,.5] implied by
constraint foo with x == 3.

19980714
  Upon encountering end-of-file after at least one variable declaration
but no objective or constraint declarations, print "No variables used"
rather than "No variables declared".

19980717
  Do not terminate compound commands when a command, such as solve,
incurs infeasibility (or other) warnings from presolve.
  Have the solve command set solve_result_num to 299 (rather than 200)
when presolve determines that the problem is inconsistent.

19980716
  Minor cosmetic tweak: when echoing new suffix declarations issued by a
solve or solution command, only put an extra newline in front of the
first echoed suffix declaration.

19980722
  For symbolic suffixes, binary or integer applies to the _num value.
  New builtin suffixes .no and .sno: if foo[i] is _xxx[j],
then foo[i].no = j (for xxx in {"con", "var", "obj"}); similarly,
if foo[i] is _sxxx[j] then foo[i].sno = j, and foo[i].sno = 0
if the solver does not see foo[i].
  Permit "display _obj.user_defined_suffix;" and
"display _sobj.user_defined_suffix;".

19980730
  Record settings of $linelim and $substout in .nl files, and adjust
these options, if necessary, when reading solutions.

19980905
  Adjust "display x;", where x is an indexed variable, so it does not
run presolve.

19980912
  Correction to 'Adjust "display x;"' in 19980904 above; if x was a
defined variable, sometimes it was not recomputed.

19981009
  Permit ":= {}" in set declarations.
  Permit set expressions of the forms
	if test then {} else set_expr
and
	if test then set_expr else {}
with the set_expr supplying the dimen (arity) of the empty set {}.

19981014
  Permit "display p.result;" for indexed collections of problems p.
  For infeasibility detected by presolve, have the solution, solve, and
write commands update the 'result' suffix of the current problem.

19981109
  New builtin params recording execution times (where possible):
	_solve_time = _solve_user_time + _solve_system_time
record CPU seconds for the most recent solve command, and
_solve_elapsed_time records wall-clock seconds for the most recent
solve; prepending _total gives sums of the corresponding entities
for all solve commands; changing "solve" to "shell" gives analogous
values for shell commands.  And
	_ampl_time = _ampl_user_time + _ampl_system_time
and _ampl_elapsed_time give analogous times for AMPL itself, excluding
times for shell and solve commands.
  On all systems, the elapsed time params are computed by the time()
function, which has a granularity of whole seconds.  The new CPU time
params report true CPU seconds on Unix and Windows NT systems (with
Win32 binaries, such as
ftp://netlib.bell-labs.com/netlib/ampl/student/mswin/samplnt.exe.gz).
On Windows 9x or 3.x, and on MS-DOS systems, CPU times are not
available; the system times are set to 0, and user times are elapsed
(wall-clock) seconds since the start of execution -- with a finer
granularity than the time() function.

19981214
  Adjust error message for a symbolic data-section value for a numeric
parameter.  For example, the old message
	type['HOST'] cannot be #ENDPOINT
is now
	type['HOST'] must be numeric, not 'ENDPOINT'
  Adjust "objective;" command so it does not try to instantiate the
current problem unless there is a partially dropped array of objectives
(in which case ampl must worry about which subscripts are valid).
  "Invisible" change: put suffix information ahead of nonlinear
information in .nl file, so .nl reading can be influenced by suffixes.

19990112
  Ignore gratuitous reset commands.
  Modified linearization of piecewise-linear terms.  To get the
linearization that had been the default since 19980309, specify
	option pl_linearize 3;

19990120
  New addition to reserved-word list: table.

19990304
  ftp://netlib.bell-labs.com/netlib/ampl/student/mswin/samplnt.exz:
Under W9x, treat '/' as '\' in the names of programs invoked by the
shell and solve commands.  (This already worked under NT, but Microsoft
does not tolerate much consistency among its operating systems.)

19990421
  Try to avoid leaving temporary files behind in the face of various
signals, such as SIGINT and SIGHUP.
  Have
	printf "%q",'';
print '' rather than nothing.
  New builtin symbolic param _cd is set to the current directory
initially and each time the cd command is executed.
  New variant -bs (and -brs) of command-line option -b for GUIs
that want to run solvers themselves.  Ask dmg for details.
  Have "option solver_msg 0;" suppress messages that appear when
presolve determines the solution (as well as the solution_message
returned by a solver).

19990607
  Table and out-arg extensions, to be described later.

19990804
  When an input file ends with a #comment not terminated by a newline,
assume a newline after the comment.
  Permit the close command to mention a comma-separated list of files
to close.
  Change to filename syntax: disallow commas in unquoted filenames.
This permits one to say things like
	load lib1, lib2;
and
	close file1.stuff, file2.xyz;
without treating the "," as part of the file name.
  Change to $ampl_include: previously it was a white-space separated
list of directories in which to look for files mentioned by "include",
"model", and "data"; now spaces are allowed within directory names
(but not before or after them), and directory names in $ampl_include
should be separated by newlines (or tabs, but tabs invite confusion;
for example, X-Windows turns tabs into spaces).
  Change $AMPLFUNC to behave similarly to $ampl_include: directory
and file names may contain internal blanks, and $AMPLFUNC now specifies
a sequence of zero or more file or directory names from which to
import functions.  Each name is first treated as the name of a shared
library (or DLL -- suffixes, such as .dll, must be explicit); if a
library by that name cannot be opened, the name is treated as a
directory name, and "/amplfunc.dll" (or "\amplfunc.dll" under MS
Windows) is appended to obtain the name of a library to load.
  The solve command now adjusts $ampl_funclibs to be a newline-
separated list of full pathnames of libraries from which functions
in the current problem instance were imported.
  New commands
	load	[libname [, libname ...] ];
	unload	[libname [, libname ...] ];
	reload	[libname [, libname ...] ];
load, unload, or reload shared libraries (from which functions and
table handlers are imported).  When at least one libname is mentioned
in the load and unload commands, $AMPLFUNC is modified to reflect the
full pathnames of the currently loaded libraries.  The reload command
first unloads its arguments, then loads them.  This can change the
order of loaded libraries and affect the visibility of imported
functions: the first name wins.  With no libname arguments, "load;"
loads all the libraries currently in $AMPLFUNC; "unload;" unloads
all currently loaded libraries, and "reload;" reloads them (which
might be useful if some have been recompiled).
  New and adjusted system sets:
	_LIB = currently loaded libraries;
	_AVAILFUNCS = currently available imported functions
			(which still must be declared before use)
	_AVAILFUNC2 = (available function, library) pairs
			(permits seeing which library is currently
			supplying an available imported function)
	_SFUNCS = imported functions that "solve;" currently uses.
  New command
	remove [filename [, filename ...]];
closes and removes the files mentioned.  "remove;" does nothing.
Exception: "remove ($log_file);" just truncates (removes and
then reopens) $log_file, much as "close ($log_file);" does.
  Extensions of funcadd.h for (still to be described) table handlers;
the updated funcadd.h is now in /netlib/ampl/solvers.

19990818
  MSDOS and Win32 "ampl" binaries: add expansion of * and ? in command-
line arguments, and replace command-line arguments of the form @filename
with the contents of file filename.
  Win32 "ampl" binaries: recognize quoted white space in command-line
arguments, no matter who provides the command line (e.g., another
program calling CreateProcess).  White space may be quoted with either
single or doubled quotes (' or ").  Within a quoted string, the quote
character may appear if it is doubled (as in AMPL strings).

19991130
  Win32 binaries only: adjust the cd command so sequences like
	cd c:/some/place/other/than/the/starting/directory;
	cd d:;
	print 'something' >'c:zot';
	shell;
will show the adjusted current directories to spawned processes
(in "solve" and "shell" commands) and will interpret 'c:zot' correctly
(as c:/some/place/other/than/the/starting/directory/zot).  Note that
under DOS, W9x, and NT, you can use / in place of \ in file names.
[For reasons known only to Microsoft, under W9x, a spawned command.com
gets its current directory information in some other way than do Win32
programs, so "shell" commands under W9x only reflect cd commands for
the current drive.  This problem goes away if you set option COMSPEC
to a Win32 command processor.]

20000128
  New command-line option -vi[nnn] to specify whether stdin should be
treated as interactive.  The default is to choose based on isatty():
if stdin and stderr both appear to be a terminal, i.e., isatty(0) and
isatty(2) both return 1, assume stdin is interactive.  Invoke
	ampl -v?
for a summary of all -v options; the description of -vi is
	-vi[nnn] {interactive mode?  (Must be first.)
		   nnn = 0 ==> no, nnn = 1 ==> yes but no prompts,
		   nnn = 2 ==> yes with prompts,
		   nnn = 3 ==> 0 or 2, based on isatty()}
In non-interactive mode, syntax errors now inhibit execution of
commands and terminate reading of the current file; previously,
commands were sometimes executed.  (Block input mode, used by AMPL Plus
and sample GUIs in http://www.ampl.com/ampl/GUI/expermt.html, remains
interactive.)
  New default presolve behavior: simplifications that appear to convert
nonlinear expressions to linear expressions really do so.
  Change to option linelim: "option linelim 0" and "option linelim 1"
work as before, while "option linelim 2" and "option linelim 3"
suppress the new conversion of "nonlinear" linear expressions
resulting from presolve simplification to true linear expressions,
and otherwise behave like "option linelim 0" and "option linelim 1",
respectively.

20000216
  Increase the resolution of _solve_elapsed_time, _shell_elapsed_time,
and "option randseed 0;".  The latter now changes more quickly.
  Warn about "fix x := 3;" when x is a subscripted variable.
  New builtin LOCAL suffix .relax:  for integer or binary variables,
.relax > 0 indicates that the integrality of the variable should be
ignored.
  Turn <<0;0,0>>x into 0.

20000217
  Arrange for integer_variable.relax values >= 2 to cause solvers to
see the variable as continuous, but for presolve to treat it as an
integer variable.  Also, have nonzero $relax_integrality to take
precedence over .relax suffix values, so
	option relax_integrality 1;	# ignore integrality everywhere
and
	option relax_integrality 2;	# use integrality in presolve,
					# but tell solvers all variables
					# are continuous
operate as heretofore, regardless of .relax settings.
  For "let" assignments to variable.declared_suffix, do not require
the "collect" phase.

20000228
  Under MS Windows, allow for spaces in $TMPDIR.
  In situations where bounds on a defined variable are equivalent to
bounds on a problem variable that the solver sees, reflect bounds on
the defined variable to the problem variable and remove the constraint
that implied the bounds on the defined variable.  Example:
	var x; var y = 3*x + 2;
	s.t. ybound: 5 <= y <= 11;	# reflect into bounds on x
	minimize zot: y^2;
  Deduce bounds on defined variables in more cases.
  Temporary(?) treatment of $substout: the "4" bit of $substout,
i.e., arranging that ($substout mod 8) >= 4, suppresses today's
reflection of bounds and stronger deductions of defined-variable bounds.

20000307
  Arrange for "option presolve_warnings -1;" to suppress both the
presolve message about how many presolve messages were suppressed and
the message about which modified options might help.

20000316
  Increase the resolution of _ampl_elapsed_time, and have "reset;"
reset _ampl_elapsed_time, even when there were no declarations to
discard.  (Otherwise, "reset;" is a no-op in this case.)

20000327
  Tweak to hash-table logic to speed up reading data on some large
examples (with many symbols).
  Tweak to "option linelim 1" output: nonlinear defined variables that
have no linear terms but are used linearly in a constraint or objective
are no longer split into two parts (unless, temporarily, the "4" bit of
$linelim is on, i.e., $linelim mod 8 >= 4).
  Command-line option -L now accepts an optional integer immediately
following the "L" (no space) to specify the initial $linelim setting.
No integer ==> -L1 (the old behavior).
  Change the default option linelim from 0 to 1.

20000427
  Tweak permitting use of AuxInfo (new field ae->AI) in imported
functions.  This is available when ae->ASLdate >= 20000427.  In calls
from AMPL, ae->AI is always null, but solvers can arrange for it to
have useful values.

20000505
  Arrange for "option compl_warn 0" to suppress the warning message
about nonsquare complementarity systems and for "option compl_warn 2"
to cause nonsquare complementarity systems to be infeasible.

20000602
  Treat variables in defining declarations (var x = ...) as defined
variables even when bounds are reflected on them.

20000608
  Addition to funcadd.h: char* getenv(const char*), which lets
imported functions and table handlers access the current environment
(as modified by option, environ, and problem commands).  Only use
getenv (i.e., (*ae->Getenv)) if al->ASLdate >= 20000608.

20000615
  Fussy numeric detail: ignore case in recognizing Infinity and NaN in
data sections and in format %q, so
	printf "%q\n", 'nan';
now prints
	'nan'
rather than
	nan
This fixes a glitch with AMPL's writing of .tab values involving
symbolic parameters with values like 'infinity' and 'nan', which
should be kept as strings, not turned into numbers, much as
'123' and 123 are distinguished.

20000706
  When loading a library, arrange for subsequent unloading of the
library to happen after any at_exit() or at_reset() processing requested
directly or indirectly by addfunc().  (This matters to an experimental
facility for importing Java functions.)
  In calls on imported functions, ensure that al->dig == 0 when
al->deriv == 0.

20000814
  Recognize command-line option -b?.

20000825
  Adjust prompts to indicate "Waiting for end of line after #" and
"Saw /*; waiting for */".  To indicate the former, # is inserted at
the start of the prompt; for the latter, * is similarly inserted.
If the prompt string would otherwise be the empty string, nothing is
inserted.
  Change to builtin "sub" and "gsub" functions: treat \n as newline
(as in the "sam" editor of plan 9).  Thus the pattern (.|\n)* now matches
everything until the end of the string, including intervening newlines.
  Make variable.no independent of whether the variable is fixed.
  Adjust presolve to make more ambitious the extended presolve
iterations that occur when $presolve > 1:  when a variable is fixed,
consider implications of modified constraints involving two or more
remaining (unfixed) variables in the current extended presolve
iteration rather than the next one; and when fixing two or more
variables leads to simplifications of the same constraint, the first
with implications for one bound (lower or upper), the second for
the other, consider implications for both bounds.  (Temporarily,
option presolve_debug 3 suppresses these changes.)
  Have presolve use values of $presolve_eps, $presolve_fixeps, and
$presolve_inteps that are rounded up by function
	R(x) = if x > 0 then 1.1^ceil(log(x)/log(1.1)) else 0.
Encode the relevant values in .nl file headers and arrange for
the solution command to issue option commands if necessary to
set $presolve_eps, $presolve_fixeps, and $presolve_inteps to values
that permit correctly reading the solution.
  In addition to using option presolve_fixeps during simple presolve
reductions (those for $presolve = 1) to decide when tighter bounds
lb and ub on a variable v (lb <= v <= ub) are close enough to
consider v fixed (when ub - lb <= R($presolve_fixeps)), also use this
test when extended presolve iterations ($presolve > 1) reduce ub - lb;
in this case, v is fixed to if ub < 0 then ub else if lb > 0 then lb
else 0.  (Temporarily, "option presolve_debug 8" suppresses this.)
  New system params _presolve_eps_L and _presolve_eps_U
indicate whether changes to $presolve_eps would matter: they should
not matter if
	_presolve_eps_L <= R($presolve_eps) < _presolve_eps_U.
Similarly, new system parameters
	_presolve_fixeps_L	_presolve_fixeps_U
	_presolve_inteps_L	_presolve_inteps_U
give corresponding bounds for $presolve_fixeps and $presolve_inteps:
presolve results should not change as long as
	_presolve_fixeps_L <= $presolve_fixeps < _presolve_fixeps_U
and
	_presolve_inteps_L <= $presolve_inteps < _presolve_inteps_U.
  Rerun presolve if $constraint_drop_tol changes.  This already
happened for $presolve_eps, $presolve_fixeps and $presolve_inteps.
  Arrange for option show_stats 2 to produce the same output as
option show_stats 1, plus an additional line about $presolve settings:
report current $presolve and indicate whether a smaller value would
suffice (albeit saving no time) or a larger value might give stronger
bounds.  In the latter case, the possible improvements are sometimes
minuscule.  Two new builtin params give values that help determine the
new line:  _presolve_req is the value of $presolve needed to
reproduced the results of the last run of presolve.  (This value has
long been encoded in .nl files to facilitate correctly reading .sol
files.)  _presolve_sug = presolve_req if further presolve iterations
would not help and is otherwise either $presolve + 1 (if $presolve >
0) or 10 (if $presolve = 0).
  Values of $presolve_*eps shown in output for $show_stats = 1 or 2
are now rounded to assure changes to R($presolve_*eps).

20000906
  Arrange for redeclarations of unsubscripted sets with no := clause
to retain their values (when redeclared to be a set of the same arity).
Example:
	set S ordered; data; set S := a b c;
	display S; redeclare set S ordered;
	display S; # OK now; previously "no data for set S"

20001002
  Add "contains" to the reserved-word list.  (Later, it will be a
set-comparison operator, with A contains B <==> B within A.)

20001006
  Turn v1*sum{i in A} p[i]*v[i] into zero, where v1 and v[i] are
variables and p[i] == 0 for all i.

20001128
  ampltabl.dll:  handle .dsn files better (no longer require them
to contain DBQ= lines); no longer require DSN=... strings to contain
DBQ=...; permit DRIVER=... to specify a full connection string (as
well as DSN=...); handle driver-specific types in addition to the
ODBC standard ones.

20010129
  In iterated commands and expressions, permit {if logical_expression}
wherever indexing expressions were previously allowed.

20010130
  For completeness, permit {if logical_expression} as the indexing
in iterated reduction operators (sum, prod, min, max, exists, forall).

20010322
  Again adjust "option randseed '';" and AMPL's -s command-line option
to make it more likely that successive invocations will give different
values.  The changes are visible mainly on Microsoft systems.

20010405
  New Caution:  unless $Caution is 0, warn about numeric option
settings where part of the option string is ignored.
  In response to "solve;" after a previous command has caused
presolve to determined the solution or eliminate all variables,
if $solver_msg is nonzero (as it is by default), print solve_message
again -- it should also have appeared after the previous command,
and it explains the situation.  Example:

	var x >= 1 <= 2 := 3; var y = x + 2;
	solve; # "No variables used after elimination of defined variables."
	solve; # now gives the same message, rather than silence.

20010507
  In imported (user-defined) functions, have printf("%s",0) print
"<NULL>" rather than fault.
  When presolve determines a feasible solution, set
solve_result_num = 99.

20010816
  Sample ampltabl.dll for MS Windows:  when reading tables in an
explicitly specified file, use the full pathname in "DBQ=..." to avoid
confusion with similar names known to the ODBC Data Source
Administrator.  When writing tables, the sample ampltabl.dll still
tries to use "DSN=..." if possible.  Note that to permit writing .xls
files, it's necessary for the ODBC Data Source Administrator to have
a DSN (e.g., a User DSN) associated with the Microsoft Excel driver with
the "Read Only" box unchecked (under Options in ODBC Microsoft Excel
Setup).

20011119
  Ensure that A union B puts members of A first.  Usually it did, but
under some conditions the members of B came first.

20011206
  Adjust processing of (the default) "option linelim 1" so defined
variables that are not involved in the current problem will not be
affected by "option linelim 1" (which took considerable time in a
motivating example).  The temporary interpretation of the "4" bit of
$linelim, introduced 20000327, is rescinded.  Now, temporarily (for
debugging), when the "4" bit of $linelim is on, today's change to
processing for "option linelim 1" will be suppressed.

20011231
  New option presolve_assoc (default 7) affects simplification of constants
in nonlinear contexts during presolve:  sum of
	1 ==> permit using associative law on + and - operations;
	2 ==> permit using associative law on * and / operations;
	4 ==> permit using distributive law on
		const*(const*thing +- const*thing).
  Change command-line option -o to write a .nl file at the end of the last
command-line input file if a "solve;" command would write a new .nl file.
(Note that if no command-line input files are given explicitly, then stdin
is treated as the single command-line input file.)

20020409
  For properly behaving solvers linked with interface library versions
>= 20020402, adjust the reading of .sol files to omit the backspaces
that used to appear when the solvers did not report any option
settings.

20020503
  Add SnprintF and VsnprintF to AmplExports, to make snprintf and
vsnprintf available to imported functions.

20020708
  Change to handling of multiple objectives:  previously objectives
were always reordered so that nonlinear objectives came first.  Now
the default is not to do this reordering.  Option nl_permute is
extended to allow indicating that objectives should be reordered as
heretofore.  $nl_permute is now the sum of
	1 ==> reorder constraints
	2 ==> reorder variables
	4 ==> reorder objectives
The default value for $nl_permute remains 3.

20020716
  Obscure enhancement to handling of complementarity constraints of
the form equation complements unbounded_variable:  indicate to solvers
that the unbounded variable is associated with the equation.
Turning the "4" bit of $compl_warn on (usually by
"option compl_warn 5;") suppresses this adjustment.

20020801
  Tweak to single-step mode: accept a semicolon or white space after
the optional repetition count after "next", "skip", and "step".
  In problems with variables in singleton complementarity constraints,
let the complementarity constraints imply the variable bounds when
$var_bounds is 1 (the default), unless the variables are also matched
with other complementarity conditions.  Example:

	var x; var y;
	s.t. c1: x >= 0 complements y >= 0;
	s.t. c2: x + y = 1;
	display _varname, _var.lb, _var.ub;
	# above now shows no bounds on y

20020814
  In the display command's printing of 2D tables, permit the final ":="
part of the ": ... :=" header to exceed $display_width (to make the
output a bit more regular).  We can always recant this tweak if it
turns out to cause more trouble than it avoids.
  In set and param declarations, treat = (or ==) as a synonym for :=,
so = behaves as it does in var declarations.  For param declarations,
this is simply an extension.  For param declarations, it changes the
meaning of =, which previously implied a surprising test that the
param had the specified value.  (We believe that = was almost never
seriously used in this way, so that this change will be harmless.
If not, "option old_param eq 1;" restores the old meaning of = in
param declarations, at least for now.)
  Arrange that failed consistency checks detected during a "solve"
command will result in solve_result = '?' and solve_result_num = -1,
just as they are at the start of execution.  Previously they retained
their values from the last successful "solve".
  Change "s.t." to "subject to" in "show" and "expand" commands.

20020924
  When foo is a subscripted problem whose declaration does not specify
a subscripted environment, have "problem foo;" make foo's environment
current.

20021025
  Arrange for variables to which piecewise-linear terms are applied
to be defined by the linearizations of the terms.  On problem 14.7(d)
in the AMPL book (first edition; 17.7(d) of the second), this reduces
the numbers of variables and constraints by 1/3.  Turning the "4"
bit of $pl_linearize on, e.g., with
	option pl_linearize 5;
suppresses this change.
  Infer bounds of 0 and 1 on binary variables, even when they are
relaxed by assignment of 1 to their .relax suffix.

20030204
  Cut over to a version that works as described in the second
edition of the AMPL book, including such relatively obscure
features as new tabular forms in data sections, handling of
"in union_of_intervals" phrases in variable declarations, and
$(expr) string expressions.  Some new constraint-logic programming
features, described in INFORMS J. Computing 14#4 (2002) pp. 322-344,
are also recognized.  (Variables in subscripts are still missing.)

20030227
  New option strict_ineq_warn (default 1) determines how to handle
constraints involving a strict inequality when the constraint would
be an algebraic constraint if the comparison were changed to permit
equality (i.e., < were changed to <= or > were changed to >=):
	0 ==> quietly treat the constraint as a logical constraint;
	1 ==> print a caution and treat as a logical constraint;
	2 ==> print a warning and reject the constraint.
Before 20030204, AMPL behaved as though $strict_ineq_warn were 2.

20030319
  Fix glitches with "unload" (which did not completely unload the
indicated library) and "delete function_name" (e.g., when the
function name coincided with a builtin random function).

20030328
  Fix a fault introduced 20030319 in handling ampltabl.dll after a
reset.  Fix a glitch that sometimes caused "unaligned access" errors
on systems with 64-bit addressing.

20030331
  Adjust option relax_integrality so binary variables added to
linear nonconvex piecewise-linear terms are retained under
	option relax_integrality 1;
and are relaxed, with no suffix information about them transmitted
to the .nl file, under
	option relax_integrality 2;
Before this change, solvers might fault unless explicitly told to
ignore SOS information when $relax_integrality was 1 and the problem
had nonconvex piecewise-linear terms.

20030527
  Adjust the output of "expand _con;" and "expand _con[n];" so that
when complementarity constraints are present, they are only indicated
by a ".L" or ".R" suffix on the constraint name.  Previously, if, say,
the first constraint was a complementarity constraint, then
both "expand _con[1];", "expand _con[2];" and "expand _ccon[1];" all
produced the same output, except for the decorations ".L" and ".R" in
the constraint names shown for _con[1] and _con[2].  Similarly,
"expand _con;" showed complementarity constraints twice, distinguished
only by ".L" and ".R" decorations.  These decorations now appear after
rather than before subscripts.

20030626
  Adjust changes made, if necessary, by the "solution" command to
$presolve_eps, $presolve_fixeps, and $presolve_inteps to properly read
a .sol file corresponding to a .nl file written before changes to
those options:  multiply the R(x) value shown in the changes of
20000825 by .95 (to put the value roughly halfway between the relevant
integer powers of 1.1), then round the result to 3 significant
figures.  For example, with a suitable solver and problem instance, in

	write bfoo; solve;
	option presolve_inteps 37; solution b.sol;

the solution command now issues

	option presolve_inteps 1.04e-06;

rather than yesterday's

	option presolve_inteps 1.0950988861107401e-06;

Because of how $presolve_inteps is discretized,
"option presolve_inteps 1.04e-6" has the same effect as the default
"option presolve_inteps 1e-6".  Today's changes also correct a bug in
deciding whether to restore $presolve_inteps.  On the relatively rare
occasions when this bug bit, it might cause reading the .sol file to
fail after a surprising "option presolve_inteps..." command was
generated and presolve ran again.

20030724
  Fix a bug in handling logical constraints (a forthcoming extension):
if $auxfiles or $($solver & '_auxfiles') requested a .row file and the
problem contained both logical constraints and objectives, a fault was
likely.
  New feature related to this bug fix:  when new option
convert_logical_to_algebraic has its default value 1, constraints that
appear to be logical constraints because they are surrounded by
parentheses but that would be recognized as algebraic constraints
without the surrounding parentheses are converted to algebraic
constraints during parsing.  Specifying
	option convert_logical_to_algebraic 0;
suppresses this conversion.

20031017
  Diagnose ": =" instead of ":=" in data tables.

20040103
  Adjust linearization of a nonconvex piecewise-linear term to use
	max($pl_bigM, 2*max{b in Breakpoints} abs(b),
		if lb > -Infinity then abs(lb) else 0,
		if ub < +Infinity then abs(ub) else 0)
where $pl_bigM was previously used (with lb and ub the lower and
upper bounds on the variable that the piecewise-linear term
multiplies).  Turning the "8" bit of $pl_linearize on, e.g., by
	option pl_linearize 9;
suppresses this change, at least for now.

20040821
  Print NaN values as NaN, regardless of their sign bits.  Previously
-Nan was sometimes (incorrectly) printed.  The sign bit of a NaN is not
supposed to have any meaning.
  Add new system param NaN (which, on systems with IEEE arithmetic, has
the value NaN and in general has the value Infinity - Infinity).

20050530
  If a "read" command issued in interactive mode encounters end-of-file,
permit further interactive input.

20050702
  Note that to get correct dual values for constraints removed by presolve
when using an interior-point solver, it's necessary to set $abs_boundtol
or $rel_boundtol to a suitable positive value.  These options are not
described in the AMPL book, but are described in the entry for 19931005
of /netlib/ampl/changes.  Note also that not all changes to $abs_boundtol
and $rel_boundtol cause recomputations; AMPL records ranges of values for
these options that have the same effect and avoids recomputations for
changes that do not matter.

20060314
  For imported functions, change unsigned long args to size_t, to
permit allocating memory blocks of size >= 4GB (a mostly invisible
change; for solvers, this requires ASLdate >= 20060122).

20060430
  Arrange for presolve to turn logical constraints into algebraic
constraints (when deductions make this possible), and then to process
the new algebraic constraints.
  New builtin params:
	_nlog_algcons = number of logical constraints turned by
			presolve into algebraic constraints
	_npre_log_algcons = number of such constraints subsequently
			eliminated by presolve.
These numbers also appear in the output for "option show_stats 3;".

20060725
  Have "display" honor $display_precision when printing symbolic
parameters having numeric values.
  When "expand" or "solexpand" prints out a constant objective,
show its value as "number" or "-number" rather than "0 + number"
or "0 - number".

20060905
  When x is a variable, have "reset data x;" discard x's current
value (and, as before, permit new default data for x's initial value
in a data section).  If x has a random default expression, it gets
resampled when x is next needed.  Previously, "reset data x" worked
this way until x was assigned a value by "let" or "solve", after which
x retained the assigned value.

20060912
  New option csvdisplay_restrict:  with $csvdisplay_restrict at its
default value, an undocumented restriction on csvdisplay is henceforth
relaxed.  Specifying "option csvdisplay_restrict 1;" enforces the
restriction in question, namely that csvdisplay issues an error
message (meant to help debug GUIs that use csvdisplay) when asked to
display more than one table.  Example:
	set A{i in 1..4} = i .. i^2;
	csvdisplay A;	# used to be disallowed
	option csvdisplay_restrict 1;
	csvdisplay A;	# gives the old behavior:
	# complains "csvdisplay would emit 4 tables."

20061005
  Fix glitches with imported functions:
1.  If-then-else expressions in constraints and objectives with imported
functions appearing only in the "then" or "else" expressions could result
in a complaint about unavailability of the functions.  They might be
available to the solver, so no complaint should appear.  Example:

	function exact;
	param p symbolic;
	var x >= 0 <= 10;
	minimize zot: (if num(p) == p then p else exact(p))*x;
	data;
	param p := '1234567890123456789';
	write 0;	# "Can't invoke unavailable function exact."

2.  If an if-then-else expression in an argument to an imported functions
evaluated by the solver had a non-constant numeric "then" expression and
a string-valued "else" expression or vice versa, confusion resulted in
that a numeric expression might appear where a symbolic one was expected.
Example:

	function goo; var x;
	minimize zot: x*goo(x, if x >= 3 then 'abc' else x+10);
	# new diagnostic: "solve" or "write" now says
	#   Cannot convert a numerical "then" or "else" to string
        #   in if-then-else expressions that solvers must evaluate.

20061007
  Fix bug in handling chains of comparisons involving more than 2
comparisons.  Example:

	print{i in 1..2}: if 1 < i < i^2 > 3 then 'yes' else 'no';
	# printed "no", then faulted

20061023
  Fix an obscure bug that gave rise to error messages such as
	presolve has k = 167168, P.nfc = 166781
  Tentatively introduce option presolve_logfile with possible values
"filename" to write file filename anew each time presolve runs;
">>filename" to append to filename each time presolve runs; and
"-" to write to the standard output file (stdout).  When
$presolve_logfile is not "" (its default value), each time presolve
updates a bound, the constraint and variable involved and updated bound
are written to $presolve_logfile.
  New builtin symbolic param _table_errmsg records the last error message
from the most recent "write table" or "read table" command, and is set
to "" if the last such command had no error.
  New option table_errbreak determines whether "write table" and/or
"read table" commands report errors and terminate processing of
commands, or suppress error reports and simply record the last error
in _table_errmsg.  $table_errorbreak (default 0) is the sum of

	1 {suppress error reporting for "write table"}
	2 {suppress error reporting for "read table"}
	4 {for iterated "read table" and "write table" commands,
		do all iterations despite errors}

Hitherto, $table_errbreak = 4 was the old behavior; now the default
value 0 stops iterated table commands at the first error.

20061030
  Fix a glitch in the display command's use of $n to stand for the
heading of column n:  sometimes the heading was '$n' rather than
simply $n (unquoted), but the quotes were not accounted for, causing
the headings not to line up with the columns beneath them.  Now the
erroneous quotes are gone.  Example:

	set A; set B;
	param p{A,B};
	data;
	set A := 'pumpkin pie' 'ice cream' 'chocolate sauce'
		 'vanilla cream' 'apple pie' strawberries;
	set B := 'New York' 'San Francisco'
		 'San Diego' 'Washington, D.C.';
	param p : 'New York' 'San Francisco'
		  'San Diego' 'Washington, D.C.' :=
	'pumpkin pie'		12	13	14	15	
	'ice cream'		22	23	24	25
	'chocolate sauce'	32	33	34	35
	'vanilla cream'		42	43	44	45
	'apple pie'		52	53	54	55
	strawberries		62	63	64	65;
	
	option display_width 40;
	display p;
	# heading ended with "'$1'   '$2'   '$3'   '$4' :="
	# instead of with    "$1   $2   $3   $4 :="

20061102
  Fix another bug with recursive set definitions.  A fault
revealed the bug, but wrong results without a fault might
sometimes have been possible.

20061121
  Add messages distinguishing "Solution determined by presolve" from
"All variables fixed" and "All relevant variables fixed".  (In the
latter case, some unused variables are not fixed.)  All these cases
still get solve_result_num = 99.
  Have main() call exit rather than returning, to prevent a fault
on an odd version of Linux.

20061130
  Fix glitch with suffixes on random variables (a forthcoming addition).

20070222
  Fix bug apparently introduced 20061005 in calling user-defined
functions:  numeric dummy arguments, such as i in "f({i in 1..3} i)",
were passed to solvers as strings rather than numbers.

20070301
  Fix glitch in "read" command introduced 20060314 that could cause an
immediate "end of file" error message.
  Tweak to $csvdisplay_restrict: when $csvdisplay_restrict mod 2 == 1,
provide a more detailed error message when csvdisplay would emit more
than one table, unless $csvdisplay_restrict mod 4 == 3, in which case
the former, one-line error message appears.

20070312
  Fix bugs in handling variables declared "in Set_expression":
integer and binary were not always properly distinguished when such
variables could be turned into integer or binary variables; with
"option presolve 0", the "in Set_expression" was ignored; and at one
point in presolve, a test for reducing the domain of a variable
using the "in Set_expression" was applied too soon, possibly causing
variables to be fixed inappropriately.  Examples giving rise to these
troubles had the form

	var x in {0, 1, 2};	# or in {0, 1} for the third bug
	var y >= 1, <= 3;
	maximize obj: y;
	subject to r: y <= 2 * x;

20070317
  Fix a bug with option single_step:  if an error (such as a bad
subscript) was reported during single-stepping, things were sometimes
not properly cleaned up, possibly resulting in a subsequent fault.
  Addition to error messages for commands:  the current _cmdno (the
count of commands executed) is now reported unless input is from the
standard input.  For the standard input, setting option stdin_offset
to 1 causes the input offset and _cmdno to be reported in error
messages for commands.
  New option single_step_cmdno causes $single_step to be set large
enough to cause single stepping when _cmdno == $single_step_cmdno.
This provides a way to repeat the execution (after "reset;" or a fresh
invocation) and enter single-step mode just before an error, to permit
looking at current values, which might shed light on the error.

20070410
  Add "	7	rand	random variable in current problem"
to $astatus_table.

Fix bug in variable.astatus when there are unused defined variables.
Example:

	var x;
	var y;
	var u = 4*x + x^2;	# unused
	var z = 3*x^2 + y^2;
	var w;
	minimize zot: z + w;
	s.t. bletch: exp(z) <= 4.2;
	display _varname, _var.astatus;
	# showed "sub" rather than "unused" for u

20070505
  Fix bug during "reset;" with handling at_exit() registrations by
imported functions:  they were lost.
  Fix bug in handling "table" declarations appearing before a suffix
declaration (including implicit suffix declarations that result from
"solve" commands in which the solver returns a new suffix), or that
appear before certain "delete" commands:  confusion, such as a fault
or surprising error message, could result.  Example:
	set I = 1..3;
	var p{I};
	table pt OUT 'pt.tab': {i in I} --> [eye] , p[i];
	suffix zork;
	write table pt;	# "unexpected type..."
  Fix bug with multiple suffix declarations appearing after some
other declarations and a delete command:  a fault or other confusion
was possible.

20070518
  Fix a bug in presolve's handling of variables declared
"in Set_expression".  Such set expressions in general are unions
of intervals (with point values included as degenerate intervals).
If presolve deduced a bound that would eliminate more than one
interval, it botched a bound update.  Example:
	var x in {0, 2, 4, 8};
	s.t. bletch: x <= 2;	# the bug changed 2 to 4
	maximize obj: x;

20070522
  Fix handling of string expressions in "commands (...);" commands
so ... can involve params declared with "= value" but not yet used
in a command.

20070612
  Tweak printing of sets under complicated conditions, such as error
messages involving next() and prev() under obscure conditions in
which the sets are now shown as a list of elements, enclosed in
braces.  Previously the braces were omitted and the elements printed
one per line, which was confusing.

20070618
  Fix the bug noted in the second "solve" comment of
	var x;
	minimize f: func(x);
	let x := ...;
	solve;	# with no primal or dual values returned, e.g., gjh
	redeclare minimize f: another_func(x);
	solve;	# value assigned by "let" to x not transmitted.

20070712
  Fix bug in handling imported functions that specify a domain for
INOUT arguments with "..." indexing.  For example,
	function f(Reals IN, ...(Reals INOUT));
encountered the bug when invoked, but
	function f(Reals IN, ...(INOUT));
did not.
  Fix a bug in simplifying certain complex logical expressions that
led to the error message "unexpected type 0x11e03 in lsimplify()".
  Fix a bug with named problems that led to the error message
"named_genall called".
  Fix a bug with "delete" and "purge":  when applied to variables,
constraints and objectives, they did not update problem declarations,
which could have led to a fault or other confusion.

20070802
  Fix a bug (infinite loop) in "redeclare set ..." when neither the
old or new set is subscripted.  Example:

	set S; var x{S} >= 0 <= 1;
	set F = {i in S: x[i] in (0,1)};	#error message
	redeclare set F;			# looped

  Fix a glitch with "show p;" for a param p that had data supplied
in a data section.  Example:

	param p; data; param p := 1.7; show p; # mentioned "random"

  Fix a performance bug:  reduce overhead when params and sets without
constraints on their values are updated.

20070825
  Fix a fault in handling "ordered by ..." in certain situations where
... is a set expression, such as a subscripted set.

20070903
  In the breakpoint and slope lists of piecewise-linear expressions,
i.e., the ... of <<...; ...>>, permit iterating parenthesized lists
of expressions, as in function calls.  Here is an example that is
now acceptable, but formerly elicited a syntax error message:

	var x in [-15,15];
	minimize zot: <<{i in 1..3} i^2; {i in 1..2} (-1,1)>> x;

20070923
  Permit Infinity (as well as NaN) as values in data sections.
  Disallow deleting some suffixes not previously noted as system
suffixes.
  System suffix stage was quietly introduced a while ago, for use with
random variable declarations (a forthcoming extension).  Permit explicit
declaration of suffix stage if no random variables have been declared,
which disallows subsequent declarations of random variables and permits
using stage as an ordinary declared suffix.

20071007
  Adjust computations behind "Too much memory used" message to make
the message more accurate when sbrk(0) proves to be unreliable.

20071111
  Fix bug in handling variables unused in the current problem when
defined variables (also unused) were present that depended on the
unused variables, the unused variables and their defined variables
were mishandled, such that commands involving them might see incorrect
values.  Example:

	var x;
	var g = x^3 - x^2 - x - 1;
	var gp = 3*x^2 - 2*x - 1;
	let x := 2;
	display x, g, gp; # OK so far
	let x := x - g/gp;
	display x,g;	# botched values

(It is more appropriate to use "param" rather than "var" in this
example; doing so would have avoided the bug.)
  Fix a bug with redeclarations:  a fault was sometimes possible.
Example:

	set I;
	param p{I};
	var f;
	param q;
	minimize zot: sum{i in I} (f*q^i - p[i])^2;
	s.t. zotc{i in I}: p[i] == f*q^i;
	delete zot;
	set J;
	redeclare s.t. zotc{i in J}: p[i] == f*q^i; # fault

20071121
  Fix bug with "ordered by":  a fault was possible under the "right"
conditions, as in this example:

	set A ordered; param p{A};
	set S ordered by A default{i in A: p[i] == 0};
	data; param :A: p := a 0  b 1  c 0  d 2;
	display S; # faulted

20071222
  Banish an obscure error message "unexpected subtype 23 in sforce".
  Fix fault with "let Set := (expression involving the Set)", e.g.,

	set A default {};
	for{i in 2..4} {
		let A := A union {j in i .. i^2: j in i+2..i+3 diff A};
		display A;
		}

20080102
  When a constraint implies a bound on a variable and integrality of
the variable causes a further adjustment to the bound, do not treat
the constraint as tight when inferring dual values.  Before this change,
constraints eliminated by presolve were sometimes given nonzero dual
values, even though they were slack.
  Tweak ODBC table handler so (e.g.) with MS SQL Server it will choose
a better type for numeric data.

20080128
  Fix a bug giving an erroneous "no data" message in the following:

	set I  = 1..3;
	minimize zot;
	var x{i in I} integer >= 0 obj zot i;
	set S;
	s.t. zap: sum{i in  S} x[i] = 1;
	let {i in 2..3} x[1].relax := 1;	# no data for S
	let S := 1..2;
	solve;

20080202
  Fix another bug giving an erroneous "no data" message under some
circumstances when a defined variable is involved in column
generation.  Example:

	set I  = 1..3;
	minimize zot;
	var x{i in I} integer >= 0 obj zot i;
	var y obj zot 3 = x[3] + 1;
	set S;
	s.t. zap: sum{i in  S} x[i] = 1;
	let {i in 2..3} x[1].relax := 1;	# no data for S
	let S := 1..2;
	solve;

  Fix a fault arising in a complicated use of defined variables.

20080207
  Fix a bug that crept in sometime in printing certain expressions,
such as iterated piecewise-linear terms.  The bug gave rise to
error message "unexpected type 0x1e600 in prex()".  Example:

    var x{i in 1..3} := i;
    display {i in 1..3} <<{j in 1..i} .5*j; {j in 0..i} (-j)^j>> x[i];

20080216
  Fix a bug (fault or worse) connected with a subscripted defined
variable whose indexing set changes size sufficiently to elicit the
bug.  This also corrects a related (so far not seen) bug with "expand"
and ".astatus" of partially dropped constraints.

20080219
  Fix a bug in handling iterated slopes and breakpoints in
piecewise-linear terms.  When it bit, the bug led to a surprising
error message, or worse.

20080306
  In interactive mode with stdin on the current include stack, have
"include 'unavailable'" return to the command prompt (to reading stdin)
when file 'unavailable' does not exist.
  Fix a bug (fault) in printing the error message "bad subscript
(use before def?) for defined var ..." under the "right" conditions.

20080307
  New variant -Rw of command-line option -R (described in change-log
entry for 19970814) disables output redirections ("> filename" and
">> filename").

20080312
  Fix bug in expressions involving the dual value of a subscripted
constraint after earlier subscripts of the constraint were dropped,
and in writing initial guesses for such constraints in .nl files.
  New option ampl_libpath specifies a sequence of directories,
one per line (analogous to $ampl_include) in which to look for
libraries mentioned in "load" commands.  When $ampl_libpath is
empty or contains only white space, the "load" command looks in
the current directory.  (The "unload" command looks for names
mentioned in previous "load" commands and quietly ignores names
not found; "reload" is a combination of "unload" and "load", so
it honors $ampl_libpath when doing its "load".  Note that
$AMPLFUNC and the builtin sets _AVAILFUNCS, _AVAILFUNC2, _LIBS
are updated by "load", "unload", and hence "reload".)

20080314
  Fix another bug in writing dual initial guesses to .nl files
when there are subscripted constraints, some of which have been
partially dropped.  An invalid .nl file could result.

20080423
  New builtin params _last_solno, _primal_solno, _dual_solno,
_status_solno:  _last_solno is incremented at each "solve" and
"solution" command that reads at least some solution information.
If any primal or dual variable or solver status (.sstatus) values
are read from the .sol file, _primal_solno, _dual_solno, and/or
_status_solno, respectively, is/are set to the new _last_solno value.
If "solve" results in "Solution determined by presolve" or
"Infeasible constraints determined by presolve", these parameters
are also updated.

20080616
  Adjust reporting of memory used to avoid confusion under some
64-bit versions of Linux where there can be large gaps in the
addresses returned by malloc().  This change affects the memory
usage reported with "option times 1;" and "option gentimes 1;".
  Fix a bug in handling defined variables fixed by presolve:
linear variables were sometimes reported as nonlinear.  Example:

	var x; var y = x + 1;
	var z = 2*x + y;
	s.t. c: z = 12;
	var w; minimize zot: w;
	# erroneously reported to have
	# a nonlinear variable and nonlinear constraint

20080629
  Fix a bug in use of variable values in set expressions (which can
only happen in commands):  if previous changes caused indexing of some
variables to change and nothing had since caused the "collect" phase
to run again, wrong values were sometimes used.  In the very
complicated example that led to finding this bug, an inserted
"display" changed some behavior (because it caused the "collect" phase
to run).  With the fix and "option times 1" to show phases, you will
still not see "collect" running before a command that does not need
it, but correct values are now computed.
  Further adjustment to reporting of memory used to avoid negative
values.
  Write "Solution determined by presolve" messages to stdout rather
than stderr.
  New builtin solve_result_num values:
	999 ==> error running solver (nonzero solve_exitcode);
	998 ==> nonexistent .sol file;
	997 ==> error in .sol file;
	996 ==> error in restoring state to read .sol file.
Treat any of these as exit code 1024 from the solver as far as
option solve_exitcode_max is concerned.  (The default value of 0
for $solve_exitcode_max will then cause a break in the execution
of commands.)  This applies to both "solve" and "solution" commands.
(Of course, solve_result_num cannot be set to 999 by a "solution"
command.)

20080701
  In some cases of failed "solve" and "shell" commands, report where
in what input file the error occurs.

20080703
  Tweaks for stochastic programming extensions (to be described later).

20080717
  Fix a bug introduced in version 20080616 in the handling of nonlinear
defined variables, e.g., those used in both constraints and objectives.
Example:

	set I = 1..2;
	var x{I} := 1;
	var y{i in I} = x[i]^2 + i;
	minimize zot: sum{i in I} (y[i] - 4)^2;
	s.t. foo: sum{i in I} y[i] <= 7;

20080804
  Fix a bug introduced in version 20060905 in which "update data",
like "reset data", discarded current values.  Now current values are
gain retained by "update data".  Example:

	set I = 1..2; var x{I}; param p{I};
	minimize zot: sum{i in I} (x[i] - p[i])^2;
	data; param p := 1 1  2 2;
	solve;  update data;
	data;  param p := 2 5;
	print sum{i in I} (x[i] + p[i]); # printed 6 rather than 9

New params (mainly for use in debugging carelessly written models):

	_backslash_map default 0;	# 0 ==> no mapping
					# 1 ==> map \\ to /
	_filename_case default 0;	# 0 == retain given case
					# 1 == force to lower case
	set _file_prefixmap dimen 2;
		# for (a,b) in _file_prefixmap, map prefix a to b

Note that under MS Windows, you have always been able to use / instead
of \ in file names.  Use of \ causes gratuitous unportability.

20080812
  Fix a bug in declared suffixes assigned value zero.  The bug only
bit sometimes; a reliable, simple example of the bug is elusive.

20080821
  Fix a glitch that caused mention of unused imported functions in .nl
files when expression simplification (such as multiplication by zero)
caused removal of all references to the functions.  Example:

	function ginv;  function hypot;
	set I = 1..2; var x{I};
	var y{I} >= 0;
	s.t. bletch: sum{i in I} y[i] <= 0;
	minimize foo: hypot({i in I} (x[i] - i)) + y[1]*ginv(y[2]);
	# ginv was listed as required in the .nl file,
	# even though presolve deduces y[1] == 0 and no actual
	# reference to ginv appears in  the .nl  file.

  Fix a glitch with "delete option ...".  Example:

	option zork 'some value', zork;
	delete option zork;
	option z*;	# said option zork ''; # not defined
	# after the bug fix, "option z*;" does not mention zork at all.