Be careful writing XPath expressions when the XML could havenon-existent

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Are you writing XPath expressions?

XPath is embedded in a lot of things: XQuery, Schematron, XSLT, XML Schema. If you are using XML, there is a good chance you are writing XPath expressions.

Heads up! 

Failure to take into account the following rule will result in countless headaches and hard-to-detect bugs.

	Rule: The result of evaluating an XPath expression 
	that compares a non-existent element against 
	anything is always false.

More ... http://xfront.com/Be-Careful-Writing-XPath-Expressions-Against-XML-Documents-that-may-have-Non-Existent-Elements.pdf  

/Roger