Goal:
  
  A : Type
  i : positive
  IHi : forall (j : positive) (x : A) (m : tree A),
        i <> j -> get i (set j x m) = get i m
  j : positive
  x : A
  H : xI i <> xI j
  ============================
   (get (xI i) (set (xI j) x Empty) = get (xI i) Empty)


DEBUG:Going to execute:
rewrite set_Empty