axis and sorting, or a two run task?

Given the following xml,

<wrapper>
	<qna> +
	    <topic>
	    <q>
                <a>
      
I want to sort by topic then q,
but having sorted, I want to
output  the topic only once,
then output the sorted q and a elements.

Many examples of using the following-sibling exist 
to check for first of a series, 
but I'm stumped with this one.

A solution is to look 'back' along the sort axis,
except there isn't one, and Mike Kay has stated
that the axis are document based even after the sort.
Damn.
The dirty one is to use a variable to hold the 'last output'
topic for comparison. [I'll say yuk now before DC does :-)]



Another is to do a transform to reorder qna
 elements by topic, then the 'sort axis' is the same as 
 document axis.

Can it be done on a single pass?

Any help appreciated.


       


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