Provides the interface and the classes for implementing the term frequency normalisation component in the DFR framework. The implemented classes can be used by specifying the class name in /etc/trec.models, or in property trec.model. For instance, adding a line DFRWeightingModel(P,L,2) in file /etc/trec.models tells the systems to use the PL2 model to run retrieval. P stands for class P in this package, L stands for class L in package org.terrier.matching.basicmodel, and "2" stands for class 2 in this package.