基金Aeknowledgements: This paper was supported by the National Natural Science Foundation of China (No. 60772082), the Natural Science Foundation of Hebei Province. China (No. 08M010), the Science Research Foundation of Ordnance Engineering The author gratefully acknowledges DENG Gao-ming for discussions which inspired this research, LI Hua for his advice, and the (anonymous) referees for their suggestions.