copy single node once from one location to another

My XML file is being fed to an external system, which I don't have control of. 
It adds a "specials"-tag at the end like this: (hardcoded)

<mydoc>
	<a id="ABC">
		<data>100</data>
		<data>101</data>
	</a>
	<a id="DEF">
		<data>200</data>
		<data>201</data>
	</a>
	<specials>
		<data_nr>2</data_nr>
		<data_text>300</data_text>
	</specials>
</mydoc>

I'd like to write a XSLT that moves the data_text to the corresponding index of "a", expressed in data_nr.
In the above case, /mydoc/a[2]

In the above example, I'd like the output to be

<mydoc>
	<a id="ABC">
		<data>100</data>
		<data>101</data>
	</a>
	<a id="DEF">
		<data>200</data>
		<data>201</data>
		<data>300</data>
	</a>
	<specials>
		<data_nr>2</data_nr>
		<data_text>300</data_text>
	</specials>
</mydoc>
I just copied the 300-data to the 2nd 'a', because data_nr was 2.

I know which fields to copy from, and I don't need any error checking (in case there are only one a , but data_nr=2 would never happen)

I can't change the external system, and in XSLT, I haven't figured out to copy the line once. My XSLTs copies them one for every data item, even if I match correctly.

/Thomas


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