(** Folding a function over all bindings of a tree. *)
Parameter fold:
forall (A B: Type), (B -> elt -> A -> B) -> t A -> B -> B.

Axiom fold_spec:
forall (A B: Type) (f: B -> elt -> A -> B) (v: B) (m: t A),
fold f m v =
List.fold_left (fun a p => f a (fst p) (snd p)) (elements m) v.