Checking for empty values

[Madhu Menon <madhu@xxxxxxxxxxxxxxx>]

Hello.

Forgive me for asking newbie questions, but it's only because I'm a newbie :)

I want to have my XSL stylesheet evaluate to see if a tag exists or the 
value is blank. How do I do this using <xsl:if> or <xsl:when> ?

For example, take the following schema:

<library>

<book>
	<name>Hound of the Baskervilles</name>
	<author>Sir Arthur Conan Doyle</author>
</book>


<book>
	<name>A Dying man's prayer</name>
</book>


<book>	
	<name>XSLT Programmer's Reference</name>
	<author>Michael Kay</author>
</book>

</library>

In the second record, the <author> data is not available. I want to check 
for this (or the tags being empty) and then conditionally produce the 
output. What would be the best way to do this?

Thanks for your help.

Madhu



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