If Svar is a set domain variable, it labels Svar to its first possible
value (which satisfies the constraints) by taking the largest possible
set of elements chosen from the smallest one to the biggest one. If
there are several possible instances of Svar, it creates choice points.
It backtracks on the element taken. If Svar is a ground set, nothing
happens. Otherwise it fails. If List is a possible list of set domain
variables occuring in the program, the simplest labeling procedure is: