Jan Burse, erstellt 15. Sep 2018
* This module provides unordered sets. The unordered sets are
* represented by lists [x1, .., xn]. The lists need not to be
* ordered or duplicate free. But the provided operations do not
* necessarily preserve duplicates:
* ?- union([2,3,4], [1,2,4,5], X).
* X = [3,1,2,4,5]
* ?- union([1,2,4,5], [2,3,4], X).
* X = [1,5,2,3,4]
* The realization uses a membership check based on (==)/2. As a
* result the predicates are safe to be used with non-ground terms.
* On the other hand, since this comparison is not arithmetical,
* 1 and 1.0 are for example considered different.
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* contains(E, S):
* The predicate succeeds when the set S contains the element E.
% contains(+Elem, +Set)
* remove(E, S, T):
* The predicate succeeds when the set S contains the element E
* and T is the set without the element.
% remove(+Elem, +Set, -Set)
* difference(S1, S2, S3):
* The predicate succeeds when S3 unifies with the difference of S1 by S2.
% difference(+Set, +Set, -Set)
* intersection(S1, S2, S3):
* The predicate succeeds when S3 unifies with the intersection of S1 and S2.
% intersection(+Set, +Set, -Set)
* union(S1, S2, S3):
* The predicate succeeds when S3 unifies with the union of S1 and S2.
% union(+Set, +Set, -Set)
* subset(S1, S2):
* The predicate succeeds when S1 is a subset of S2.
% subset(+Set, +Set)
* permutation(S1, S2):
* The predicate succeeds when S1 is a permutation of S2.
% permutation(+Set, +Set)