xml.instruct!
xml.urlset(
  'xmlns' => 'http://www.sitemaps.org/schemas/sitemap/0.9',
  'xmlns:xsi' => 'http://www.w3.org/2001/XMLSchema-instance',
  'xsi:schemaLocation' => 'http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd'
) do
  @urls.each do |url|
    xml.url do
      xml.loc(url.loc)
      xml.lastmod(url.lastmod)
    end
  end
end