theorem Th23: :: EUCLID_2:22
  for x1,x2 being Element of REAL n holds |(-x1, x2)| = - |(x1, x2 )|
proof
  let x1,x2 be Element of REAL n;
  len x1 = n & len x2 = n by CARD_1:def 7;
  hence thesis by RVSUM_1:122;
end;
