diff --git a/library/ordsets.yap b/library/ordsets.yap
index cf5d26497..72ee3e9e4 100644
--- a/library/ordsets.yap
+++ b/library/ordsets.yap
@@ -28,6 +28,7 @@
 	ord_intersect/2,	%  Set x Set ->
 	ord_intersect/3,	%  Set x Set -> Set
 	ord_intersection/3,	%  Set x Set -> Set
+	ord_intersection/4,	%  Set x Set -> Set x Set
 	ord_seteq/2,		%  Set x Set ->
 	ord_setproduct/3,	%  Set x Set -> Set
 	ord_subset/2,		%  Set x Set ->
@@ -170,6 +171,26 @@ ord_intersection(>, Head1, Tail1, _,     Tail2, Intersection) :-
 
 
 
+
+%   ord_intersection(+Set1, +Set2, ?Intersection, ?Difference)
+%   is true when Intersection is the ordered representation of Set1
+%   and Set2, provided that Set1 and Set2 are ordered sets.
+
+ord_intersection(L, [], [], L) :- !.
+ord_intersection([], L, [], L) :- !.
+ord_intersection([Head1|Tail1], [Head2|Tail2], Intersection, Difference) :-
+	compare(Order, Head1, Head2),
+	ord_intersection(Order, Head1, Tail1, Head2, Tail2, Intersection, Difference).
+
+ord_intersection(=, Head,  Tail1, _,     Tail2, [Head|Intersection], Difference) :-
+	ord_intersection(Tail1, Tail2, Intersection, Difference).
+ord_intersection(<, Head1,     Tail1, Head2, Tail2, Intersection, [Head1|Difference]) :-
+	ord_intersection(Tail1, [Head2|Tail2], Intersection, Difference).
+ord_intersection(>, Head1, Tail1, Head2,     Tail2, Intersection, [Head2|Difference]) :-
+	ord_intersection([Head1|Tail1], Tail2, Intersection, Difference).
+
+
+
 %   ord_seteq(+Set1, +Set2)
 %   is true when the two arguments represent the same set.  Since they
 %   are assumed to be ordered representations, they must be identical.