Return a string of component sitemap number part_number.

        Used in the case of a large list that is split into component
        sitemaps.

        basename is used to create "index" links to the sitemapindex

        Q - what timestamp should be used?

def as_xml_part(self, basename="/tmp/sitemap.xml", part_number=0):
        """Return a string of component sitemap number part_number.

        Used in the case of a large list that is split into component
        sitemaps.

        basename is used to create "index" links to the sitemapindex

        Q - what timestamp should be used?
        """
        if (not self.requires_multifile()):
            raise ListBaseIndexError(
                "Request for component sitemap for list with only %d entries when max_sitemap_entries is set to %s" %
                (len(self), str(
                    self.max_sitemap_entries)))
        start = part_number * self.max_sitemap_entries
        if (start > len(self)):
            raise ListBaseIndexError(
                "Request for component sitemap with part_number too high, would start at entry %d yet the list has only %d entries" %
                (start, len(self)))
        stop = start + self.max_sitemap_entries
        if (stop > len(self)):
            stop = len(self)
        part = ListBase(itertools.islice(self.resources, start, stop))
        part.capability_name = self.capability_name
        part.default_capability()
        part.index = basename
        s = self.new_sitemap()
        return(s.resources_as_xml(part))