From charlesreid1

Multiset data structure

Multisets can be thought of as counting dictionaries - that is, dictionaries where the keys are unique objects, and their corresponding values are counts of their frequencies of occurrence. In fact, in Python the built-in collections module provides a Counting object precisely for this purpose.

Multiset combinatorics

In mathematics, a multiset is defined formally as a 2-tuple, in which each element of a set maps to a natural number that indicates the number of occurrences of that element.

See AOCP/Multisets