def plain_inverse_document_frequency(term, numerator = 0, denominator = 0)
      log((documents.size + numerator) / (@model.document_count(term).to_f + denominator))
    end
def plain_inverse_document_frequency(term, numerator = 0, denominator = 0)
      log((documents.size + numerator) / (@model.document_count(term).to_f + denominator))
    end
@see https://github.com/reddavis/TF-IDF/blob/master/lib/tf_idf.rb#L50

 SMART t, Salton f, Chisholm IDFB