<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_string(self):
 """Convert the sitemap into a string."""

        root = etree.Element('urlset', nsmap={None: SITEMAP_NS})
        for url in self.urls:
            url.generate(root)
        return etree.tostring(root, pretty_print=True, xml_declaration=True,
                              encoding='utf-8')