Math.Combinatorics.Multiset

Just a quick note to say that I’ve just uploaded a new package, multiset-comb, to Hackage. It primarily has functions for generating all the distinct permutations and partitions of a multiset (a set where elements are allowed to occur multiple times). Efficiently generating distinct permutations and partitions of a multiset is nontrivial (generating all partitions and permutations as if all the elements were distinct, and then discarding duplicate results, is hopelessly inefficient). The permutations code is new; the partitions code is a slight generalization of the code from my article in Issue 8 of the Monad.Reader. I put this code together primarily because I need it for my combinatorial species library (look for a new release of that soon!), but it’s fairly independent, and I thought others might find it useful, so I’m releasing it as a separate library.