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.