theorem
  (fdif(f,h).1)/.x = Shift(f,h)/.x - f/.x
proof
  set f1 = Shift(f,h);
  (fdif(f,h).1)/.x = fdif(f,h).(0+1)/.x
  .= fD(fdif(f,h).0,h)/.x by Def6
  .= fD(f,h)/.x by Def6
  .= f/.(x+h) - f/.x by Th3
  .= f1/.x - f/.x by Def2;
  hence thesis;
end;
