This is a new index (based on GiST) with code name FastX, which follows different way of handling data in XML. The index stucture might be used for Xquery or Xpath 2.0 support without LibXML library. The implementation is based on XISS/R XML Indexing and Storage System Using RDBMS by Philip J. Harding, Quanzhong Li, Bongki Moon from University of Arizona, as proposed on VLDB.
There is also a related project for XML validation in PostgreSQL database management system.