Revision 3466

Added new Set object class to the Parser.  It implements a finite set
of real numbers, for use with unions and intervals.  E.g., (1,2) U {3}
or (1,2) U {3,4,5}.  You can created Set objects in your perl code via
the Set() command, e.g, Set(3,4,5) or Set("{1,2,3}").  You should set
the Context to Context("Interval") if you plan to use Set objects, as
this defined the braces to form sets (rather than using them as
parentheses, which is the default WW behavior).  Note that in Interval
context, you can NOT use braces as parentheses.

Current, Set objects are only allowed to be sets of numbers.  It would
be possible to extend that in the future.