diff --git a/library/bag.m b/library/bag.m
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+++ b/library/bag.m
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+%---------------------------------------------------------------------------%
+%---------------------------------------------------------------------------%
+%
+% file: bag.nl
+%	An implementation of multisets.
+% main author: conway.
+%
+%---------------------------------------------------------------------------%
+%---------------------------------------------------------------------------%
+
+:- module bag.
+
+:- interface.
+
+:- type bag(T).
+
+:- pred bag__init(bag(T)).
+:- mode bag__init(out) is det.
+
+:- pred bag__insert(bag(T), T, bag(T)).
+:- mode bag__insert(in, in, out) is det.
+
+:- pred bag__remove(bag(T), T, bag(T)).
+:- mode bag__remove(in, in, out) is det.
+
+:- pred bag__remove_all(bag(T), T, bag(T)).
+:- mode bag__remove_all(in, in, out) is det.
+
+:- pred bag__contains(T, bag(T)).
+:- mode bag__contains(in, in) is semidet.
+
+%---------------------------------------------------------------------------%
+%---------------------------------------------------------------------------%
+:- implementation.
+
+:- import_module map, int.
+
+:- type bag(T)		==	map(T, int).
+
+%---------------------------------------------------------------------------%
+
+bag__init(B) :-
+	map__init(B).
+
+%---------------------------------------------------------------------------%
+
+bag__insert(B0, I, B) :-
+	(
+		map__search(B0, I, C0)
+	->
+		C is C0 + 1
+	;
+		C = 1
+	),
+	map__set(B0, I, C, B).
+
+%---------------------------------------------------------------------------%
+
+bag__remove(B0, I, B) :-
+	(
+		map__search(B0, I, C0)
+	->
+		C is C0 - 1,
+		(
+			C > 0
+		->
+			map__set(B0, I, C, B)
+		;
+			map__delete(B0, I, B)
+		)
+	;
+		B = B0
+	).
+/*
+%---------------------------------------------------------------------------%
+
+bag__remove_all(B0, I, B) :-
+	map__delete(B0, I, B).
+
+%---------------------------------------------------------------------------%
+
+bag__contains(I, B) :-
+	map__contains(B, I).
+
+%---------------------------------------------------------------------------%
+%---------------------------------------------------------------------------%
+*/