code: def as_xml_index(self, basename="/tmp/sitemap.xml"):
          
          num_parts = self.requires_multifile()
          if (not num_parts):
              raise ListBaseIndexError(
                  "Request for sitemapindex for list with only %d entries when max_sitemap_entries is set to %s" %
                  (len(self), str(
                      self.max_sitemap_entries)))
          index = ListBase()
          index.sitemapindex = True
          index.capability_name = self.capability_name
          index.default_capability()
          for n in range(num_parts):
              r = Resource(uri=self.part_name(basename, n))
              index.add(r)
          return(index.as_xml())
docs: Return a string of the index for a large list that is split.
  
          All we need to do is determine the number of component sitemaps will
          be is and generate their URIs based on a pattern.
  
          Q - should there be a flag to select generation of each component sitemap
          in order to calculate the md5sum?
  
          Q - what timestamp should be used?