> I guess I was looking for the
> cleanest way to exercise this.  I guess it would be this:
> <xsl:apply-template select="not(foo/bar[@a='b'])" 
> mode="unmatchedresults"/>

No, the result of the selection has to be a set of nodes, it can't be a
boolean.

The idea in your head of invoking a template rule if something isn't there
is not a ridiculous one, but it's something that the language doesn't
support, and couldn't be made to support without introducing some new
fundamental concepts. So the best one can say is: it ain't going to happen;
find another route to your destination.

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