[ubl-ndrsc] list of XML parsers

To expand on Eve's suggestion, we should develop a standard list of
XML parsers (and other tools) that guide our conformance testing for
guidelines.  Here is a start:

Xerces-J
Xerces-C++
Apache Crimson (does this support XSD?)
Tibco Extensibility
XML Spy
W3C XSV
IBM XML Schema Checker (on alphaworks)

The last two are often suggested as the most conformant with the spec.