Definition combine (m1 m2: PTree.t L.t) : PTree.t L.t :=
  match xcombine m1 m2 with
  | Same|Same1 => m1
  | Same2 => m2
  | CC m => m
  end.

Lemma gcombine: forall m1 m2 i, opt_eq (PTree.get i (combine m1 m2)) (f (PTree.get i m1) (PTree.get i m2)).