Hmap.SOutput signature of the functor Make
module Key : sig ... endKeys.
val empty : tempty is the empty map.
val is_empty : t -> boolis_empty m is true iff m is empty.
fold f m acc folds over the bindings of m with f, starting with acc
exists p m is true iff there exists a bindings of m that satisfies p.
val cardinal : t -> intcardinal m is the number of bindings in m.