module type COLLECTION =sig..end
type 'a t
val empty : 'a t
val fold : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a
val map : ('a -> 'b) -> 'a t -> 'b t
val filter : ('a -> bool) -> 'a t -> 'a t
val filter_map : ('a -> 'b option) -> 'a t -> 'b t
val flat_map : ('a -> 'b t) ->
'a t -> 'b t