1. College of Computer Science and Technology, Jilin University, Changchun 130012, P. R. China
2. Key Laboratory of Symbolic Computation and Knowledge Engineering of Ministry of Education, Jilin University, Changchun 130012, P. R. China
3. College of Mathematics, Jilin University, Changchun 130012, P. R. China
4. College of Computer Science, Northeast Electric Power University, Jilin 132012, P. R. China