
theorem ZOR:
  for f be real-valued Function, x be object holds
  (delneg f).x = 0 or (delpos f).x = 0
  proof
    let f be real-valued Function, x be object;
B2: f.x = (delneg f).x - (delpos f).x by VAL;
    f.x = (delneg f).x or f.x = -(delpos f).x by VOR;
    hence thesis by B2;
  end;
