Make.2-M
type key
= S.elt
type +'a t
val empty : 'a t
val find : key -> 'a t -> 'a
val add : key -> 'a -> 'a t -> 'a t
val mem : key -> 'a t -> bool
val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b