theorem Th122:
  for x1,x2 being real-valued FinSequence st len x1=len x2 holds
  |(-x1, x2)| = -|(x1, x2)|
proof
  let x1,x2 be real-valued FinSequence;
  assume len x1=len x2;
  then |(-x1, x2)| = (-1)*|(x1, x2)| by Th121;
  hence thesis;
end;
