Module Monoid

module Monoid: sig .. end

module type S = sig .. end

module Int: sig .. end

module All: S  with type t = bool

module Any: S  with type t = bool

module Dual: 
functor (M : S) -> S  with type t = M.t

module Endo: 
functor (T : sig

type t 

end) -> S  with type t = T.t -> T.t

module Pair: 
functor (M1 : S) -> 
functor (M2 : S) -> S  with type t = M1.t * M2.t

module Opt: 
functor (S : Semigroup.S) -> S  with type t = S.t option