Follow these links to find related materials on this topic:
| http://xxx.lanl.gov/ps/math.AG/9805071 |
| http://xxx.lanl.gov/ps/math.AG/0003093 |
| http://xxx.lanl.gov/ps/math.AG/0003094 |
| http://xxx.lanl.gov/ps/math.AG/0106140 |
| http://xxx.lanl.gov/ps/math.AG/0107040 |