theorem
  for m,n be non zero Nat, F be PartFunc of REAL-NS m,
  REAL-NS n, G be PartFunc of REAL m,REAL n, x be Point of REAL-NS m, y be
  Element of REAL m st F=G & x=y holds F is_differentiable_in x iff G
  is_differentiable_in y;
