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 formula_1. this is sometimes called the multiset coefficient.