How to perform Xpath queries on Children Nodes ( using libxml2 )

I want to perform Xpath queries in the context of a chld node and not the document. Below I have
code similar to what I am doing. When i try:

      evaluator.setNewContext(node)
      typeA = evaluator.xpathEval("//node/contentNodeTypeA")

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the xpathquery: //node/contentNodeTypeA  is executed on the entire document.

Below you'll find the code that illustrates what I am trying to do.

<rootNode>
     <nodes>
               <node>    <contentNodeTypeA/><contentNodeTypeB/>   </node>
               <node>     <contentNodeTypeA/><contentNodeTypeB/>   </node>
               <node>     <contentNodeTypeA/><contentNodeTypeB/>    </node>
      </nodes>
</rootNode>

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Using libxml2 i do the following

import libxml2

doc = libxml2.parseFile ("doc.xml")
evaluator = doc.xpathNewContext()

nodeList  = evaluator.xpathEval("//rootNode/nodes/node")

for node in nodeList:
      evaluator.setNewContext(node)
      typeA = evaluator.xpathEval("//node/contentNodeTypeA")
      typeB = evaluator.xpathEval("//node/contentNodeTypeA")
      

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