A few more collections.

https://github.com/mtth/more-containers

More containers

• Multimap

  [...] a multimap (sometimes also multihash or multidict) is a generalization of a map or associative array abstract data type in which more than one value may be associated with and returned for a given key. -- Wikipedia

• Multiset

  [...] a multiset (aka bag or mset) is a modification of the concept of a set that, unlike a set, allows for multiple instances for each of its elements. -- Wikipedia.