docs: def to_string(self):
          """Convert SitemapIndex into a string."""
          root = etree.Element('sitemapindex', nsmap={None: SITEMAP_NS})
          for sitemap in self.sitemaps:
              sm = etree.SubElement(root, 'sitemap')
              etree.SubElement(sm, 'loc').text = sitemap.url
              if hasattr(sitemap.lastmod, 'strftime'):
                  etree.SubElement(sm, 'lastmod').text = \
                      sitemap.lastmod.strftime('%Y-%m-%d')
              elif isinstance(sitemap.lastmod, str):
                  etree.SubElement(sm, 'lastmod').text = sitemap.lastmod
          return etree.tostring(root, pretty_print=True, xml_declaration=True,
                                encoding='utf-8')