class type key_handler =object..end
method inquire_key : domain:Neturl.url option -> realm:string -> auth:string -> keyThe method is called when a new session must be authenticated.
 The domain is the URI from the request. URIs
 must have the form "http://host:port/path", i.e. the port is
 always written out. If the request doesn't have a URI, or if the
 request is directed to a proxy, domain will be None.
 The realm parameter is the realm identifier.
 In auth the name of the authentication 
 method is passed (lowercase characters). The method must
 search (or interactively ask for) a key, and return it. 
 The key must refer to one of the passed realms. 
 If the method raises Not_found,
 authentication will fail.
method invalidate_key : key -> unitThe handler is notified that authentication failed for this key