theorem
  for V, W being non empty ModuleStr over INT.Ring, f being Form of V,W
  holds f - f = NulForm(V,W)
  proof
    let V, W be non empty ModuleStr over INT.Ring, f be Form 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 BLDef7
    .= (NulForm(V,W)).(v,w) by FUNCOP_1:70;
  end;
  hence thesis;
end;
