Multiset
From AoPSWiki
A multiset is a slight generalization of the notion of a set. A set is defined by whether or not each object is an element. A multiset is defined not just by its elements, but also by how many times each element is contained. In other words, a multiset is a set where duplication of elements is allowed.
For example, 
as multisets.
Note that the order of the elements is unimportant, so 
.
