This work is supported by China National Nature Foundation. Current address: Department of Mathematics;
Shanghai Normal University;
Shanghai;
200234;
P.R. China. I would like to thank P. T. Johnstone;
A;
Jung;
G.-Q. Zhang for delivering me their papers;
which lead me to complete this paper. I specially thank P. T. Johnstone for his help when he visited Shaanxi Normal University in June;
1994. But foremost;
I express my deep feeling of gratitude to my doctoral advisor;
G.-J;
Wang;
whose gentle guidance gave me the necessary amount of orientation whilst leaving me considerable freedom in conducting my research;