code: def cat(self):
          
          categories = self._series.categories
  
          if categories.are_numeric:
              return parse_xml(
                  self._numRef_cat_tmpl.format(**{
                      'wksht_ref':     self._series.categories_ref,
                      'number_format': categories.number_format,
                      'cat_count':     categories.leaf_count,
                      'cat_pt_xml':    self._cat_num_pt_xml,
                      'nsdecls':       ' %s' % nsdecls('c'),
                  })
              )
  
          if categories.depth == 1:
              return parse_xml(
                  self._cat_tmpl.format(**{
                      'wksht_ref':  self._series.categories_ref,
                      'cat_count':  categories.leaf_count,
                      'cat_pt_xml': self._cat_pt_xml,
                      'nsdecls':    ' %s' % nsdecls('c'),
                  })
              )
  
          return parse_xml(
              self._multiLvl_cat_tmpl.format(**{
                  'wksht_ref': self._series.categories_ref,
                  'cat_count': categories.leaf_count,
                  'lvl_xml':   self._lvl_xml(categories),
                  'nsdecls':   ' %s' % nsdecls('c'),
              })
          )
docs: Return the ``<c:cat>`` element XML for this series, as an oxml
          element.