binding variables to values in XPath

[Zeljko Velaja]

I'm reading in the XPath language specification that the lexical production 
$QName retrieves the value of the QName variable. Yet I cannot find anything in 
the XPath specification to declare a new variable and assign a value to it 
within an XPath expression. Did I miss something in the specification or does 
Xalan define a language extension wich should be usefull in what I mean to do ?