
theorem
  for V, W being non empty ModuleStr over INT.Ring, f being FrForm of V,W
  holds f - f = NulFrForm(V,W)
  proof
    let V, W be non empty ModuleStr over INT.Ring, f be FrForm of V,W;
    now
      let v be Vector of V, w be Vector of W;
      thus (f-f).(v,w) = f.(v,w) - f.(v,w) by Def7
      .= (NulFrForm(V,W)).(v,w) by FUNCOP_1:70;
    end;
    hence thesis;
  end;
