The authors thank Dr. T. Amari for his valuable comments and suggestions on this paper. W.T.J. is grateful to Prof. Ai Guoxiang for his considerations, and to Mrs. Paula Fulmer for her help in English language. This work was supported by NSFC grants 19573012 and 19791090.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999