> I understand (correctly???) that I can do
> 
> <xsl:variable name="x" as="schema-element(a)">
>   <xsl:apply-templates select="//foo"/>
> </xsl:variable>
> 
> 
> The question is, what happens if I run that on a schema-aware 
> processor, 
> and the <apply-templates.../> evalutates to something which is not a 
> valid a element? 

You will get a type error.

> can I assume that the variable at run-time is 
> really bound to a valid a? And will its descendants be valid, too?

Yes. That's the whole point of the type declaration: it's an assertion about
the type of the value, and the processor guarantees to fail if the assertion
is not true.

Michael Kay
http://www.saxonica.com/