theorem Th19:
  for f1,f2,f3 being complex-valued Function holds
  (f1 - f2)(#)f3 = f1(#)f3 - f2(#)f3
proof
  let f1,f2,f3 be complex-valued Function;
  (-f2)(#)f3 = -(f2(#)f3) by Th17;
  hence thesis by Th15;
end;
