The two-tier design in the rear serves two functions to make the whole more layered. Firstly, the progressive layered sense brings potential energy of the whole forward from the side vision and meanwhile it integrates the top lid with intelligent module visually. Secondly, the curved surface of intelligent module is gradually fading out to the front lid, which integrates the top lid with the back as well, with the potential energy pointing to the front. The application of these two features contributes to the strong sense of layer and also integration.