theorem
  for V, W being non empty ModuleStr over INT.Ring, f being Form of V,W
  holds f + NulForm(V,W) = f
  proof
    let V, W be non empty ModuleStr over INT.Ring, f be Form of V,W;
    set g = NulForm(V,W);
    now
      let v be Vector of V, w be Vector of W;
      thus (f+g).(v,w) = f.(v,w) + g.(v,w) by BLDef2
      .= f.(v,w) + 0 by FUNCOP_1:70
      .= f.(v,w);
    end;
    hence thesis;
  end;
