The Path of Control
http://www.xml.com/pub/a/2005/05/04/tr-xml.html

DuCharme, Bob
XML.com
2005

Abstract:

Covers XPath's new ability to do some things that every real programming language can do: conditional statements and iteration, or, as they're more colloquially known, "if" statements and "for" loops. We'll also look at a useful related technique for checking whether certain conditions do or don't exist in a set of nodes.