Integration with XML SiteMap

Sitemaps for search engines robots help robots to find their way to fresh/or major content on a web site.

One of the way to the tell robots where to find the sitemap file is to use the robots.txt file

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Specifying the Sitemap location in your robots.txt file

You can specify the location of the Sitemap using a robots.txt file. To do this, simply add the following line:

Sitemap:

The should be the complete URL to the Sitemap, such as: http://www.example.com/sitemap.xml

This directive is independent of the user-agent line, so it doesn't matter where you place it in your file. If you have a Sitemap index file, you can include the location of just that file. You don't need to list each individual Sitemap listed in the index file.
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from http://www.sitemaps.org/protocol.php#submit_robots

It would be nice to dynamically update the robots.txt content when XML Sitemap module is installed
http://drupal.org/project/xmlsitemap

This could be an optionnal behavior since some webmaster prefer not to reveal the name of their sitemap.