DiscreteIntervals and DiscreteIntervalUnions

[Hugh Hoover <heh>]

I really really really really really want these 2 region classes to have
a mostly common protocol that is slightly larger than DiscreteRegion.
Most especially, I want them both to have lowerBound and upperBound messages,
and that I be able to declare a (single) class that has these.
  We can either have (another) common superclass for these, or make one
a subclass of the other.  I think I prefer the latter, and suggest that
DiscreteIntervalUnion be a subclass of DiscreteInterval.  It seems that the
union is a reasonably natural extension of the basic interval.  The main
distinguishing factor between them would be isSimple(), and what you got
back from the simpleRegions message.

complaints?

--Hugh