theorem
  for A, B being non empty set, f being Function of A, B, x being
  Element of A, y being set holds f+*(x,y).x = y
proof
  let A, B be non empty set, f be Function of A, B, x be Element of A, y be
  set;
  x in A;
  then x in dom f by FUNCT_2:def 1;
  hence thesis by Th30;
end;
