https://en.wikipedia.org/wiki/Information_retrieval
= Lucene =
* https://en.wikipedia.org/wiki/Lucene
= Lucy =
* [http://stackoverflow.com/questions/32604657/apache-solr-vs-apache-lucy Apache solr vs Apache Lucy]
= Xapian =
* [DebianPackage:xapian-bindings], [DebianPackage:xapian-core]