SWI-Prolog -- library(ordsets)

is_ordset/1	True if Term is an ordered set.
list_to_ord_set/2	Transform a list into an ordered set.
ord_add_element/3	Insert an element into the set.
ord_del_element/3	Delete an element from an ordered set.
ord_disjoint/2	True if Set1 and Set2 have no common elements.
ord_empty/1	True when List is the empty ordered set.
ord_intersect/2	True if both ordered sets have a non-empty intersection.
ord_intersect/3	Intersection holds the common elements of Set1 and Set2.
ord_intersection/2	Intersection of a powerset.
ord_intersection/3	Intersection holds the common elements of Set1 and Set2.
ord_intersection/4	Intersection and difference between two ordered sets.
ord_memberchk/2	True if Element is a member of OrdSet, compared using ==.
ord_selectchk/3	Selectchk/3, specialised for ordered sets.
ord_seteq/2	True if Set1 and Set2 have the same elements.
ord_subset/2	Is true if all elements of Sub are in Super.
ord_subtract/3	Diff is the set holding all elements of InOSet that are not in NotInOSet.
ord_symdiff/3	Is true when Difference is the symmetric difference of Set1 and Set2.
ord_union/2	True if Union is the union of all elements in the superset SetOfSets.
ord_union/3	Union is the union of Set1 and Set2.
ord_union/4	True iff ord_union(Set1, Set2, Union) and ord_subtract(Set2, Set1, New).