sig
type key0 = Variable.t
type key1 = Variable.t
type 'a t
val create : int -> int -> 'a t
val mem : 'a t -> key0 -> key1 -> bool
val find : 'a t -> key0 -> key1 -> 'a
val set : 'a t -> key0 -> key1 -> 'a -> unit
val remove : 'a t -> key0 -> key1 -> unit
end