code: def _lvl_xml(self, categories):
          
          def lvl_pt_xml(level):
              xml = ''
              for idx, name in level:
                  xml += (
                      '                  <c:pt idx="%d">\n'
                      '                    <c:v>%s</c:v>\n'
                      '                  </c:pt>\n'
                  ) % (idx, escape('%s' % name))
              return xml
  
          xml = ''
          for level in categories.levels:
              xml += (
                  '                <c:lvl>\n'
                  '{lvl_pt_xml}'
                  '                </c:lvl>\n'
              ).format(**{
                  'lvl_pt_xml': lvl_pt_xml(level),
              })
          return xml
docs: The unicode XML snippet for the ``<c:lvl>`` elements containing
          multi-level category names.