val fold: (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
val filter: (key -> 'a -> bool) -> 'a t -> 'a t
val bindings: 'a t -> (key * 'a) list
val fold: (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
val filter: (key -> 'a -> bool) -> 'a t -> 'a t
val bindings: 'a t -> (key * 'a) list