Paper is at http://ir.dcs.gla.ac.uk/terrier/publications/fulltext_plachouras.pdf
Vassilis discussed two approximations of the multinomial model, which were called ML2 and ML_
2. The latter of these used Lancos approximation instead of using Stirling series to calcualte the factorials in ML2.
Ok. I have recoded to work in new version of fields. These will be committed to core in TREC-160.