Have you heard about multisets? It's a basic data structure and there is some fun you can have with it!
Check out this talk from Scala Love Conference by Rúnar Bjarnason.
Counts, Combinations, Dice, and D&D
This talk introduces multisets, a basic data structure that’s sadly missing from the standard Scala library. We’ll look at some fun applications, interesting generalizations, and delve a little into the mathematics.