Not exactly a *debugging* technique, but one of the principles of XP
(extreme programming) is that extensive unit tests do reduce the amount
of debugging.

In the case of XSLT, XSLTunit can be used to define unit tests on
patterns (both named and unamed). See http://xsltunit.org for details.

Hopes this helps,

Eric
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Eric van der Vlist       http://xmlfr.org            http://dyomedea.com
(W3C) XML Schema ISBN:0-596-00252-1 http://oreilly.com/catalog/xmlschema
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