module type S : sig ... endtype key
type 'a t
val empty : 'a tval add : key:key ->
data:'a -> 'a t -> 'a tval find : key -> 'a t -> 'aval remove : key -> 'a t -> 'a tval mem : key -> 'a t -> boolval iter : f:(key:key -> data:'a -> unit) ->
'a t -> unitval map : f:('a -> 'b) -> 'a t -> 'b tval mapi : f:(key -> 'a -> 'b) ->
'a t -> 'b tval fold : f:(key:key -> data:'a -> 'b -> 'b) ->
'a t -> init:'b -> 'b