From the readme:
Lux is an open source XML search engine formed by fusing two excellent technologies: the Apache Lucene/Solr search index and the Saxon XQuery/XSLT processor.
At its core, Lux provides XML-aware indexing, an XQuery 1.0 optimizer that rewrites queries to use the indexes, and a function library for interacting with Lucene via XQuery. These capabilities are tightly integrated with Solr, and leverage its application framework in order to deliver a REST service and application server.
The REST service is accessible to applications written in almost any language, but it will be especially convenient for developers already using Solr, for whom Lux operates as a Solr plugin that provides query services using the same REST APIs as other Solr search plugins, but using a different query language (XQuery). XML documents may be inserted (and updated) using standard Solr REST calls: XML-aware indexing is triggered by the presence of an XML-aware field in a document. This means that existing application frameworks written in many different languages are positioned to use Lux as a drop-in capability for indexing and querying semi-structured content.
The application server is a great way to get started with Lux: it provides the ability to write a complete application in XQuery and XSLT with data storage backed by Lucene.
If you are looking for experience with XQuery and Lucene/Solr, look no further!
May be a good excuse for me to look at defining equivalence statements using XQuery.
I first saw this in a tweet by Michael Kay.