in "A Standard for Robot Exclusion", at
http://www.robotstxt.org/wc/norobots.htmls
Webmasters can use the /robots.txt file to forbid conforming robots
from accessing parts of their web site.
The parsed files are kept in a WWW::RobotRules object, and this object
provides methods to check if access to a given URL is prohibited.
The same WWW::RobotRules object can be used for one or more parsed
/robots.txt files on any number of hosts.
