Goal:
  
  A : Type
  i : positive
  IHi : forall (j : positive) (m : tree' A),
        i <> j -> get i (rem' j m) = get' i m
  j : positive
  m : tree' A
  H : xI i <> xO j
  ============================
   (get (xI i) (Node (rem' j m) None Empty) = None)


DEBUG:Going to execute:
rewrite gNode; auto