Multiset
A multiset is a collection of elements that don't have any particular order, but can occur several times (unlike a regular set).
val add : 'a -> 'a t -> 'a t
val remove : 'a -> 'a t -> 'a t
val mem : 'a -> 'a t -> bool
val find : 'a -> 'a t -> int
val of_list : 'a list -> 'a t