reserve a,b,s,t,u,lambda for Real,
  n for Nat;
reserve x,x1,x2,x3,y1,y2 for Element of REAL n;

theorem :: EUCLID_2:23
  for x1,x2 being Element of REAL n holds |(x1, -x2)| = - |(x1, x2)| by Th23;
