Revision 2594

Fixes for handling intervals better.  In particular, deal better with
ambiguous situations, like (1,2) that could be a point.

You can use [a,a] to specify the single point a.  I'd like to make {a}
represent this (and maybe even {a,b,c} to form finite sets), but this
would interfere with the use of { and } as alternative parentheses.
(Does anyone really use these?)

Do we need a cross product for intervals (and sets) as a means of
representing regions in R^2 (or higher)?