Goal:
  
  A : Type
  i : positive
  x : A
  IHi : forall (m : tree' A) (j : positive),
        i <> j -> get' i (set' j x m) = get' i m
  m1 : tree' A
  m2 : tree' A
  j : positive
  H : xI i <> xI j
  ============================
   (get' i (set' j x m2) = get' i m2)


DEBUG:Going to execute:
apply IHi; congruence