def _find_child(self, tag):
        """Find the child C{etree.Element} with the matching C{tag}.

        @raises L{WSDLParseError}: If more than one such elements are found.
        """
        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]
Find the child C{etree.Element} with the matching C{tag}.

        @raises L{WSDLParseError}: If more than one such elements are found.