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_partial_differentiable_in x,i iff
  G is_partial_differentiable_in y,i;
