> Of course it is trivial to write a program to do the test, but I was hoping > to find a fully declarative solution, so that our formal requirements > [http://www.language-archives.org/OLAC/protocol.html] can be accompanied by > one or more XML documents which more or less transparently implement them. >> I would be grateful for any advice about how to test for set membership > using an XML validation language. You should take a look at Schematron [1] which is an validation language implemented using XSLT. I presented a paper at XSLT-UK provides some addtional background [2]. Schematron provides the kinds of constraints checking that you require, and can be used in conjunction with other schema languages. For example the topologi schema tool [3] is capable of taking a W3C XML Schema which contains embedded Schematron rules and applying both sets of constraints. Let me know if you need any help. Cheers, L. [1]. http://www.ascc.net/xml/resource/schematron/schematron.html [2]. http://www.bath.ac.uk/~ccslrd/papers/schematron_xsltuk.html [3]. http://www.topologi.com -- Leigh Dodds, Research Group, Ingenta | "Pluralitas non est ponenda http://weblogs.userland.com/eclectic | sine necessitate" http://www.xml.com/pub/xmldeviant | -- William of Ockham