I shared sometime back on this blog, on the work I am doing regarding XML Schema 1.1 assertions support in Xerces-J. The XML Schema 1.1 assertions processing requires a XPath 2.0 processor for performing Schema validation.
The Xerces-J team has opted to use the open source XPath 2.0 processor, PsychoPath. PsychoPath was developed by Andrea Bittau and his team. The PsychoPath team donated the PsychoPath code base to Eclipse community, where it is now formally used in the Eclipse, Web Tools Platform project. Future enhancements to PsychoPath are now taking place at Eclipse.
Since Xerces-J is using PsychoPath XPath 2.0 engine, we wish that PsychoPath be ideally 100% compliant to the XPath 2.0 specification, so Xerces-J users can use much of the failities of the XPath 2.0 language while using XML Schema 1.1 assertions.
After looking at the PsychoPath source code and using it quite a bit, my personal observation is, that PsychoPath has a pretty good XPath 2.0 implementation. Please refer to this documentation for knowing more about PsychoPath and the current compliance status.
The Eclipse WTP team is working actively to solve any remaining non-compliant items in PsychoPath. Incidentally, I have been working recently to help improve PsychoPath's compliance to the XPath 2.0 spec, and have contributed few patches to Eclipse.
We are also planning to run the W3C XPath 2.0 test suite on PsychoPath, and targetting PsychoPath to pass the test suite, with 100% coverage. This should give the PsychoPath adopters more confidence while using it.