How can I find out the parent of a select child from XSL?

for example, I have the following XML :

	<a>Test</a>
		<b>Test2</b>
		<b2>Test3</b2>
	</a>

if in my code user selected node <b>, how can i detect the parent of <b>;
which in this case is node <a>?.  Basicly i want to change the display of
<a> when someone select <b>

many thanks
joe


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