message output if elements/attributes don't exist

[Jens von Pilgrim <j....@4flow.de>]

Hello,

I'm wondering if there exists a quick solution for displaying messages
if a searched element doesn't exist.

If an XSLT queries an XML source and searches for elements that doesn't
exists, no error occurs. E.g.

<source>
	<p>hello</p>
</source>

xslt:

 <xsl:value-of select="source/p/foo" />

The foo element doesn't exists and the xslt produces an empty string.
But what I need is, at least, a message or warning. 
Is there a quick solution or do I have to use the debugging API? In the
latter case, do you have a sample code showing me messages if an element
(or attribute) doesn't exists?

Regards,

jens


--
Jens von Pilgrim
4flow AG, Berlin, Germany
http://www.4flow.de