Set is a non-empty FD set representing the domain Min..Max \/ Rest,
where Min..Max is a non-empty interval (see fdset_interval/3)
and Rest is another FD set (possibly empty).
If Max is sup, then Rest is the empty FD set. Otherwise, if Rest
is non-empty, all elements of Rest are greater than Max+1.
This predicate should only be called with either Set or all other
arguments being ground.