Definition combine: forall (A B C: Type), (option A -> option B -> option C) -> t A -> t B -> t C := PTree.combine.

Theorem 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).