RPN calculator

Hello,
I'm trying to do a simple calculator with XSL.
This means that I would like to transform something like (but I can change
the source format, this is only an example):
<multiply>
	<arg1>2</arg1>
	<arg2>3</arg2>
</multiply>
This should yeld 6, and it's easy.
But I would also like to do:
<multiply>
	<arg1>
		<sum>
			<arg1>1</arg1>
			<arg2>2</arg2>
		</sum>
	</arg1>
	<arg2>3</arg2>
</multiply>
...and so on, recursively. As with an RPN calculator.
This should yeld (1+2)*3=6.

Can anybody point me in the right direction? How can I do this?

Thank you,

Costantino

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