**5. ****Multiset** - **Wikipedia**
**Link:** https://en.wikipedia.org/wiki/Multiset

**Description:** In mathematics, 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.The positive integer number of instances, given for each element is called the multiplicity of this element in the **multiset**. As a consequence, an infinite number of multisets exist, which contain only elements a and b, but vary by the ...