transforming one specific subtree only

[apologies in advance if this was dealt with on xsl-list already; I couldn't 
find it on the FAQ, and the archive isn't searchable.  I also consulted Mike 
Kay's book, but it was two'o'clock in the morning, and there even his book was 
of no use ;-).]

I have a document of type

<a>
	<b ID="b1">...</b1>	
	<b ID="b2">...</b1>	
	<b ID="b3">...</b1>	
	<b ID="b4">...</b1>	
</a>

In my stylesheet I have a global param $nodeId defined which gets assigned an 
ID from an argument sent to the processor, such as "b2".

I would like to only transform the subtree whose root has the ID stored in 
$nodeId, and ignore, ie. not produce any output at all for all other elements.  
If no matching ID is found, some boilerplate text should be produced instead.

To explain:  The (docbook) document is a userguide that serves at the same time 
as an online help system.  That is, somebody clicks on the contextual help 
button on the web page, which will trigger the above-described process, 
extracts the corresponding subtree, transforms it and displays the result in a 
help window.

Any help on this probably trivial question is appreciated.  Apologies again,
Jakob Fix.

PS:  cc to me appreciated, I'm on xsl-list-digest, and it's rather urgent.  


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