Log Message: 
Added ability to subtract intervals, sets and unions. Adjusted the
precedence of the union 'U' to be above _ and + so that things like
(1,5) U (7,10)  {8} U (2,3) will do ((1,5) U (7,10))  ({8} U (2,3))
rather than the previous (1,5) U ((7,10)  {8}) U (2,3). Finally,
added a constant 'R' to the Interval context that is equivalent to
(inf,inf), so you can do things like R{0} now.
Still need to work out reducing unions so that things like (1,3)U(2,4)
can become (1,4).
