"Group by" question

Hi :)

I would like to kwonw how can I do a "group by".. I explain:
Imagine i have the next xml:
<docs>
	<doc id="doc1" address="av. A1">
		<pages>
			<page>...</page>
			<page>...</page>
		</pages>
	<doc>
	<doc id="doc2" address="av. A2">
		<pages>
			<page>...</page>
		</pages>
	<doc>
	<doc id="doc3" address="av. A1">
		<pages>
			<page>...</page>
			<page>...</page>
			<page>...</page>
		</pages>
	<doc>
</docs>

I need a xsl to "group by" the address. Like this:

<address val="av. A1">
	<doc id="doc1">
		...
	</doc>
	<doc id="doc3">
		...
	</doc>
</address>
<address val="av. A2">
	<doc id="doc2">
		...
	</doc>
</address>

Tks in advanced :)

Hilder Sousa