code: def _find_child(self, tag):
          
          tag = self._get_namespace_tag(tag)
          children = self._root.findall(tag)
          if len(children) > 1:
              raise WSDLParseError("Duplicate tag '%s'" % tag)
          if len(children) == 0:
              return None
          return children[0]
docs: Find the child C{etree.Element} with the matching C{tag}.
  
          @raises L{WSDLParseError}: If more than one such elements are found.