WL#2549: improve query expansion

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

1. take into account the relevance of found documents, assigning different
weight to additional words. take  few least-matching documents, using their
words with a negative weight

2. from "improving automatic query expansion" - take into accound word
correlations (that is f(idf(w1),idf(w2),idf(w1,w2))), rerank with taking window
into accound and
fuzzy boolean query.