[Gecode] Some more set quirks...

[Christian Schulte]

As I moved almost all iterators to their right place, I happened to notice
some other things:
 
 - What used to be SeqUN is now called NaryAppend (as that's what it is
doing). In the process of moving the authorship info got lost. The author
might want to add himself to the header.
 - What used to be COnstRange is now called Singleton (as that's hwat it
is). The old implementation was buggy in case the singleton was actually
empty!
 - I noticed that sometimes set difference is expressed in terms of
complement and intersection. Aargh. Use the Iter::Ranges::Diff instead!
 - The only thing to be moved is complement. I can do that provided some
kind soul has a look at its width method as it appears to be errourneous. 

That's all
Christian 

--
Christian Schulte, http://www.imit.kth.se/~schulte/