I am pleased to announce the first release of signed-multiset, which implements an abstract datatype for multisets with negative membership. The package can be obtained from http://hackage.haskell.org/package/signed-multiset-0.1 As always, feedback is welcome. Cheers, Stefan signed-multiset-0.1 ------------------- Multisets (or bags) are sets in which elements may occur more than once. The number of times an element occurs in a multiset is called its multiplicity. This package provides an efficient implementation of so-called signed multisets, which generalise multisets by allowing for negative membership. That is, elements in a signed multiset can have negative multiplicities. See also: Wayne D. Blizard. Negative membership. Notre Dame Journal of Formal Logic/, 31(3):346--368, 1990.