public function setResource($resource = null)
        {
            if (null === $resource || is_string($resource) ||
                $resource instanceof Acl) {
                $this->_resource = $resource;
            } else {
                require_once 'Zend/Navigation/Exception.php';
                throw new Exception(
                        'Invalid argument: $resource must be null, a string, ' .
                        ' or an instance of Acl');
            }

            return $this;
        }