CCMultiSet.MakeSourcemodule O : Set.OrderedTyperemove_mult set x n removes at most n occurrences of x from set.
update set x f calls f n where n is the current multiplicity of x in set (0 to indicate its absence); the result of f n is the new multiplicity of x.
union a b contains as many occurrences of an element x as count a x + count b x.
meet a b is a multiset such that count (meet a b) x = max (count a x) (count b x).
intersection a b is a multiset such that count (intersection a b) x = min (count a x) (count b x).
MultiSet difference. count (diff a b) x = max (count a x - count b x) 0.