protected function validate_pobject($pobject) {
        if (!is_object($pobject)) {
            throw new convert_path_exception('convert_path_no_object', get_class($pobject));
        }
        if (!method_exists($pobject, $this->get_processing_method()) and
            !method_exists($pobject, $this->get_end_method()) and
            !method_exists($pobject, $this->get_start_method())) {
            throw new convert_path_exception('convert_path_missing_method', get_class($pobject));
        }
    }