theorem
  cD(f,h).x = Shift(f,h/2).x - Shift(f,-h/2).x
proof
  cD(f,h).x = (cdif(f,h).1).x by DIFF_3:16
    .= Shift(f,h/2).x - Shift(f,-h/2).x by DIFF_1:25;
  hence thesis;
end;
