Multiset 
A multiset is an unordered collection of values. They are also known as bags.
Unlike sets where each value is either included or not, multisets permit duplicates. Consequently, they're useful for maintaining a count of distinct values.
See the documentation of HashMultiSet for examples.
License
Licensed under either of
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)
at your option.
Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.