@hackage signed-multiset0.2

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 (also known as hybrid sets or shadow sets), 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.