module CCMixset:sig..end
      let k1 : int key = newkey () in
      let k2 : int key = newkey () in
      let k3 : string key = newkey () in
      let set =
        empty
        |> set ~key:k1 1
        |> set ~key:k2 2
        |> set ~key:k3 "3"
      in
      assert (get ~key:k1 set = Some 1);
      assert (get ~key:k2 set = Some 2);
      assert (get ~key:k3 set = Some "3");
      ()
    type t 
type 'a key 
'a in a setval newkey : unit -> 'a keynewkey () creates a new unique key that can be used to access
    a 'a value in a set. Each key created with newkey is distinct
    from any other key, even if they have the same type.
    Not thread-safe.
val empty : tval set : key:'a key -> 'a -> t -> tset ~key v set maps key to v in set. It means that
    for every set, get ~key (set ~key v set) = Some v.val get : key:'a key -> t -> 'a optionget ~key set obtains the value for key in set, if any.val get_exn : key:'a key -> t -> 'a
val cardinal : t -> int