Frequency maps are often the most natural way of expressing multi-sets in Clojure, eg. {{{:a 3 :b 2}}} is a set (unordered) of 3 {{:a}}'s and 2 {{:b}}'s: {{[:a :a :a :b :b]}}. Unfortunately, math.combinatorics currently only accepts sets in the latter form ie. a collection of possibly repeated values. It would be nice if math.combinatorics functions either supported sets represented as frequency maps or had versions which did.