code: def xpath_eval(self,expr):
          
          ctxt = common_doc.xpathNewContext()
          ctxt.setContextNode(self.xmlnode)
          ctxt.xpathRegisterNs("muc",self.ns.getContent())
          ret=ctxt.xpathEval(to_utf8(expr))
          ctxt.xpathFreeContext()
          return ret
docs: Evaluate XPath expression in context of `self.xmlnode`.
  
          :Parameters:
              - `expr`: the XPath expression
          :Types:
              - `expr`: `unicode`
  
          :return: the result of the expression evaluation.
          :returntype: list of `libxml2.xmlNode`