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 xelements_oh : forall (m1 m2: t A) (o: option A) (i : key) (v: A), List.In (xO i, v) (xelements (Node m1 o m2) xH) -> List.In (i, v) (xelements m1 xH).