theorem
  Hom(C) c= V implies (hom-?(V,a)).f = [[Hom(cod f,a),Hom(dom f,a)],hom(
  f, a ) ]
proof
  assume Hom(C) c= V;
  hence (hom-?(V,a)).f = (hom-?(a)).f by Def25
    .= [[Hom(cod f,a),Hom(dom f,a)],hom(f,a)] by Def21;
end;
