|Function: Tries to get a session identifier from the cookies (or alternatively from the GET parameters) sent by the client (see SESSION-COOKIE-NAME and SESSION-COOKIE-VALUE). This identifier is then checked for validity against the REQUEST object REQUEST. On success the corresponding session object (if not too old) is returned (and updated). Otherwise NIL is returned. A default method is provided and you only need to write your own one if you want to maintain your own sessions.