collections_extended is a pure Python module with no dependencies
providing extra collections. The new collections include bags AKA
multisets, setlists AKA unique lists or ordered sets, a bijection,
a RangeMap which is a mapping from ranges to values, and an IndexedDict
class. There are also frozen (hashable) varieties of bags and
setlists.
PR: 287126
Approved by: acm (mentor)
6b35562463