get node value by passing node name dynamically in Xpath

I am using Tibco xpath.

my inputs are like

<output>
<outputf1>123</outputf1>
<outputf2>123</outputf2>
<outputf3>123</outputf3>
<outputf4>123</outputf4>
</output>

Outputf1, outputf2 ..comes dynamically from another activity.

how to retrieve the value by passing the node name dynamically to the XML.