This class implements the DFR BM weighting model, which is an approximation of
BM25 in the DFR framework.
Feel free to combine the DFR_BM model with any frequency normalisation method.
However, it is NOT recommended to use DFR_BM with the first normalisation for
after effect. For example, to use the DFR_BM model with the normalisation 2,
add the following line in file etc/trec.models:
DFRWeightingModel(DFR_BM, , 2) Leave the space between the comas blank so
that the first normalisation is disabled.