Partly true.

The use-when expression is evaluated at compile time. The use-when
expression can invoke the element-available() function. When the
element-available() function is called in the use-when expression then it is
evaluated at compile time. If element-available() is called in other
expressions then it is (conceptually) evaluated at run-time. However, in all
cases except pathological test cases invented by malicious QA people, the
argument to element-available() is a string literal, which means the result
of the expression is known statically.

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

> -----Original Message-----
> From: James Fuller [mailto:jim.fuller@xxxxxxxxxxxxxx] 
> Sent: 30 June 2005 10:05
> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> Cc: ElektonikaMail@xxxxxxxxxxxxxxxxxxxxxx; exslt@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re:  Re: [exslt] Re:  Importing compiled 
> stylesheets
> 
> Colin Paul Adams wrote:
> 
> >>>>>>Mike, what you say is true, but is not relevant to the case.
> >>>>>>xsl:use-when IS a compile-time construction - it runs 
> at compile-time, 
> >>>>>>not at run-time.
> >>>>>>and the condiotion is not a runt-ime condition - 
> element-available
> >>>>>>evaluates, AT COMPILE TIME, whether or not the instruction is
> >>>>>>available.
> >>>>>>            
> >>>>>>
> 
> being very lazy at the moment,
> 
> is this behavior what the spec explicitly states or is it 
> implementator
> intepretation...also is it consistent between XSLT 1.0 and XSLT 2.0 ?
> 
> cheers, Jim Fuller