(** Applying a function pairwise to all data of two trees. *)
Parameter combine:
forall (A B C: Type), (option A -> option B -> option C) -> t A -> t B -> t C.

Axiom gcombine:
forall (A B C: Type) (f: option A -> option B -> option C),
f None None = None ->
forall (m1: t A) (m2: t B) (i: elt),
get i (combine f m1 m2) = f (get i m1) (get i m2).