Multiset
A multiset (sometimes called a bag) is a concept from mathematics. In many ways, multisets are like sets. Certain items are either elements of that multiset, or they are not. However, multisets are different from sets: The same type of item can be in the multiset more than once. For this reason, mathematicians have defined a relation (function) that tells, how many copies of a certain type of item there are in a certain multiset. They call this multiplicity. For example, in the multiset { a, a, b, b, b, c }, the multiplicities of the members a, b, and c are 2, 3, and 1, respectively. From a set of n elements, the number of r-element multisets is written as . This is sometimes called the multiset coefficient.