How to create a temporary xml tree nd then re use it

Hello,
Can I create a temporary xml tree in memory and then re-use it.
Like 
<load-qty>12</load-qty>
<load-qty>2</load-qty>
<load-qty>53</load-qty>
Say I read the document and create a new element in memory called <qty>12</qty>
for each element from xml file

and then run sum(qty)? on this newly created tree

Thanks in advance

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