Just another update

Task/Set/Prolog/set-2.pro

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+%%  Set creation
+
+?- list_to_ord_set([1,2,3,4], A), list_to_ord_set([2,4,6,8], B).
+A = [1, 2, 3, 4],
+B = [2, 4, 6, 8].
+
+%% Test m ∈ S -- "m is an element in set S"
+
+?- ord_memberchk(2, $A).
+true.
+
+%% A ∪ B -- union; a set of all elements either in set A or in set B.
+
+?- ord_union($A, $B, Union).
+Union = [1, 2, 3, 4, 6, 8].
+
+%% A ∩ B -- intersection; a set of all elements in both set A and set B.
+
+?- ord_intersection($A, $B, Intersection).
+Intersection = [2, 4].
+
+%% A ∖ B -- difference; a set of all elements in set A, except those in set B.
+
+?- ord_subtract($A, $B, Diff).
+Diff = [1, 3].
+
+%% A ⊆ B -- subset; true if every element in set A is also in set B.
+
+?- ord_subset($A, $B).
+false.
+
+?- ord_subset([2,4], $B).
+true.
+
+%% A = B -- equality; true if every element of set A is in set B and vice-versa.
+
+?- $A == $B.
+false.
+
+?- $A == [1,2,3,4].
+true.
+
+%% Definition of a proper subset:
+
+ord_propsubset(A, B) :-
+    ord_subset(A, B),
+    \+(A == B).
+
+%% add/remove elements
+
+?- ord_add_element($A, 19, NewA).
+NewA = [1, 2, 3, 4, 19].
+
+?- ord_del_element($NewA, 3, NewerA).
+NewerA = [1, 2, 4, 19].