theorem
  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
A1: len x1=len x2;
  then len (-x2)=len x1 by Th114;
  then |(-x1, -x2)| = -|(x1, -x2)| by Th122
    .= --|(x1, x2)| by A1,Th122;
  hence thesis;
end;
