Set manipulation (not in ISO)

• is_set(Set):   True if Set is a proper list without duplicates.

• eliminate_duplicate(List, Set):   True when Set has the same element as List in the same order. The left-most copy of the duplicate is retained.

• intersection(Set1, Set2, Set3):   True if Set3 unifies with the intersection of Set1 and Set2.

• union(Set1, Set2, Set3):   True if Set3 unifies with the union of Set1 and Set2.

• subset(SubSet, Set):   True if all elements of SubSet belong to Set as well.

• subtract(Set, Delete, Result):   Delete all elements from Set that occur in Delete (a set) and unify the result with Result'