Storing calculated values in global variables ?

Hello,

I am a new user of XSLT and am not very good with it. Can anyone please
help me with this problem ? If I have the following 2 XML Documents

Doc1.xml
<?xml version="1.0"?>
<Testing>
	<Range>
		<From>500</From>
		<To>300</To>
		<From>300</From>
		<To>400</To>
		<From>600</From>
		<To>1000</To>
	</Range>
</Testing>

Doc2.xml
<?xml version="1.0"?>
<Testing>
	<Range>
		<From>100</From>
		<To>300</To>
		<From>200</From>
		<To>400</To>
		<From>700</From>
		<To>1000</To>
	</Range>
</Testing>

I can sort the elements from both the documents in ascending or
descending order and extract the maximum and minimum value elements
using a single stylesheet. But how do I perform mathematical operations
on the sorted elements ? For e.g if I want to add the first and the last
<From> element from the sorted list of <From> elements how can I do this
and store this value in a global variable so that it can be reused ? 

Thanx,
Chitra


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