1. Department of Mathematics, Statistics and Insurance, The Hang Seng University of Hong Kong, Siu Lek Yuen, Shatin, Hong Kong, China
2. Department of Statistics and Actuarial Science, The University of Hong Kong, Pokfulam, Hong Kong, China
3. Department of Statistics, The Chinese University of Hong Kong, Shatin, Hong Kong, China
4. School of Mathematics, Statistics and Actuarial Science, The University of Kent, Canterbury CT2 7NZ, UK