theorem
  fdif(f,h).1 = fD(f,h)
proof
  fdif(f,h).1 = fdif(f,h).(0+1) .= fD(fdif(f,h).0,h) by DIFF_1:def 6
    .= fD(f,h) by DIFF_1:def 6;
  hence thesis;
end;
