A high weight in tf–idf is reached by a high term frequency (in the given document) and a low document frequency of the term in the whole collection of documents
# Term frequency–Inverse document frequency for each term
#
# A high weight in tf–idf is reached by a high term frequency
# (in the given document) and a low document frequency of the term in the
# whole collection of documents
var tfidf: Vector = terms_count is lazy
lib/vsm/vsm.nit:365,2--370,40