let V, W be non empty ModuleStr over INT.Ring ; :: thesis: for f being FrForm of V,W holds f - f = NulFrForm (V,W)
let f be FrForm of V,W; :: thesis: f - f = NulFrForm (V,W)

now :: thesis: for v being Vector of V
for w being Vector of W holds (f - f) . (v,w) = (NulFrForm (V,W)) . (v,w)

let v be Vector of V; :: thesis: for w being Vector of W holds (f - f) . (v,w) = (NulFrForm (V,W)) . (v,w)
let w be Vector of W; :: thesis: (f - f) . (v,w) = (NulFrForm (V,W)) . (v,w)
thus (f - f) . (v,w) = (f . (v,w)) - (f . (v,w)) by Def7
.= (NulFrForm (V,W)) . (v,w) by FUNCOP_1:70 ; :: thesis: verum

end;

hence f - f = NulFrForm (V,W) ; :: thesis: verum