Return XML serialization of this list.

        This code does not support the case where the list is too big for
        a single XML document.

def as_xml(self):
        """Return XML serialization of this list.

        This code does not support the case where the list is too big for
        a single XML document.
        """
        self.default_capability()
        s = self.new_sitemap()
        return s.resources_as_xml(self, sitemapindex=self.sitemapindex)