Extended version of OCaml's maps
module type MonadS = sig ... end
module type S = Stdlib.Map.S
module type ExtS = sig ... end
module Make
(M : Stdlib.Map.OrderedType) :
ExtS
with type key = M.t
and type 'a t = 'a Stdlib.Map.Make(M).t
and module Set := Stdlib.Set.Make(M)