val mem : 'a key -> t -> bool
val add : 'a key -> 'a -> t -> t
val singleton : 'a key -> 'a -> t
val rem : 'a key -> t -> t
val find : 'a key -> t -> 'a option
val get : 'a key -> t -> 'a
val iter : (binding -> unit) -> t -> unit
val fold : (binding -> 'a -> 'a) -> t -> 'a -> 'a
val for_all : (binding -> bool) -> t -> bool
val exists : (binding -> bool) -> t -> bool