20250930
  Put quotes around strings that look like numbers in an error message
about inappropriate set members.  Example:
	set S ordered by Reals;
	data; set S := 1 '1' 3 '3' 2 '2';
	display S;
now gives
	S contains '1', which is not
	within Reals;

  New set expression

	sort indexing by expr

where "indexing" is an indexing set (enclosed in braces)
and the expression "expr" may involve dummy variables
given in the indexing set.  For example, suppose we have

	set I;
	var x{I};
	set J;

and other declarations; after a "solve" we might

	let J := sort {i in I} by x[i];

In a different context, we might declare

	set I;
	param p{I};
	set J = sort {i in I} by p[i];

Then J will be recomputed when p changes.
The indexing set can have arity > 1, e.g., be a set of pairs.
Ties are broken by sorting the indexing elements as in "display":
elements are sorted lexicographically, with numbers compared
numerically, numbers regarded as less than strings, and strings
compared as strings (ASCII collating sequence).