[xsl] Wrap changing element sequence into container: with 'for-each-grou

Given two XML fragments

<a>
 <b1/>
 <b2/>
 <c/>
</a>

and

<a>
 <b2/>
 <c/>
</a>

I would like to wrap <b1/><b2/> into a <b/> container, and the same 
should happen to <b2/> when occurring alone. The other elements can be 
assumed to remain unchanged.
While I found quite some hints on solving grouping problems in the XSLT 
FAQ and in the XSL list archive, I can't figure out how to solve this 
wrapping task using XSLT 2.0. There is an interesting posting here: 
http://www.biglist.com/lists/xsl-list/archives/200201/msg00804.html
But I don't know how this could serve as a basis for a solution to my 
problem.
I suspect that "for-each-group" should be my friend; how would I need to 
use it here? Or is there any other, more elegant solution?
Yours,

  Yves


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