Return the ``<c:numRef>`` element specified by the parameters as
        unicode text.

def numRef_xml(self, wksht_ref, number_format, values):
        """
        Return the ``<c:numRef>`` element specified by the parameters as
        unicode text.
        """
        pt_xml = self.pt_xml(values)
        return (
            '            <c:numRef>\n'
            '              <c:f>{wksht_ref}</c:f>\n'
            '              <c:numCache>\n'
            '                <c:formatCode>{number_format}</c:formatCode>\n'
            '{pt_xml}'
            '              </c:numCache>\n'
            '            </c:numRef>\n'
        ).format(**{
            'wksht_ref':     wksht_ref,
            'number_format': number_format,
            'pt_xml':        pt_xml,
        })