Fixpoint xfind (i j : key) (m : t A) : option A :=
      match i, j with
      | _, xH => find i m
      | xO ii, xO jj => xfind ii jj m
      | xI ii, xI jj => xfind ii jj m
      | _, _ => None
      end.

Lemma xfind_left : forall (j i : key) (m1 m2 : t A) (o : option A) (v : A), xfind i (append j (xO xH)) m1 = Some v -> xfind i j (Node m1 o m2) = Some v.