theorem
  for a, b being Element of W holds
    a (--) b & a (--) a implies (a "\/" b) (--) a
  proof
    let a, b be Element of W;
    assume a (--) b & a (--) a; then
    (a "\/" b) (--) (a "\/" a) by CompDef;
    hence thesis by LemmaId2;
  end;
