Fixpoint combine (m1: t A) (m2: t B) {struct m1} : t C :=
match m1 with
| Leaf => xcombine_r m2
| Node l1 o1 r1 =>
match m2 with
| Leaf => xcombine_l m1
| Node l2 o2 r2 => Node' (combine l1 l2) (f o1 o2) (combine r1 r2)
end
end.

Theorem gcombine:
forall (m1: t A) (m2: t B) (i: positive),
get i (combine m1 m2) = f (get i m1) (get i m2).