In most positions where a set or set variable is expected one can also use a set expression. A set expression is composed from ground sets (integer lists), set variables, and the following set operators:
Set1 /\ Set2 % intersection
Set1 \/ Set2 % union
Set1 \ Set2 % difference
When such set expressions occur, they are translated into auxiliary intersection/3, union/3 and difference/3 constraints, respectively.