1. School of Mathematics and Statistics; Yunnan University; Kunming, Yunnan 650091 China
2. Department of Statistics; University of British Columbia; Vancouver, BC, Canada V6T 1Z2
3. Department of Statistics and Actuarial Science; University of Waterloo; Waterloo, ON, Canada N2L 3G1
4. School of Statistics; East China Normal University; Shanghai 200241 China