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_2:28
  for x1,y1,y2 being Element of REAL n holds |(x1, y1-y2)| = |(x1, y1)|
  - |(x1, y2)| by Th26;
