
theorem
  for V, W being non empty ModuleStr over INT.Ring, f being FrForm of V,W
  holds f + NulFrForm(V,W) = f
  proof
    let V, W be non empty ModuleStr over INT.Ring, f be FrForm of V,W;
    set g = NulFrForm(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 Def2
      .= f.(v,w) + 0.INT.Ring by FUNCOP_1:70
      .= f.(v,w);
    end;
    hence thesis;
  end;
