Every Helper can be extended by user. User's directory is HELPER_PATH. or

// create new helper
class Auth extends Thule_Helper {
......
}

// extended by current helper
class Auth extends Thule_Auth {
.........
}

How to register "reserved function"

public function islogin($hconn,$authObject,$value) {
    if($this->isReserved(__METHOD__)) { // check reserved
        return TRUE;
    }
    // your code added here
}

For example in Controllers

$this->loadHelper("login","Login");
$this->Login->islogin($hconn,$this->auth,1);

class MvcController extends Thule_Controller {
    function f1() {
        $this->loadHelper("login","Login");
        $this->Login->islogin($hconn,$this->auth,1); // should not be called ,just skip
    }
}

Helper List