How to tell the number of distinct attribute values?

Hi,  I would like to have a stylesheet that translates an XML document from:

<group>
<group>
			<item period="1998">6261</item>
			<item period="1999">6738</item>
		</group>
		<group>
			<item period="1998">2686</item>
			<item period="1999">3356</item>
		</group>
		<group>
			<item period="1998">3575</item>
			<item period="1999">3382</item>
		</group>
</group>

to:

<group>
<group period="1998">
			<item period="1998">6261</item>
			<item period="1998">2686</item>
			<item period="1998">3575</item>
		</group>
		<group period="1999">
			<item period="1999">6738</item>
			<item period="1999">3356</item>
			<item period="1999">3382</item>
		</group>
</group>

The rule is to re-group the items according to their @period value.
However, I could not figure out a way to tell how many distinct @period
values there are.

Any suggestions?

Thx,
Yufei
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