public function addPath($db_filename = null, $db_directory = null, $file = null)
    {
        if (!empty($db_filename) && !in_array($db_filename, $this->_paths['language_strings_db_filename'])) {
            $this->_paths['language_strings_db_filename'][] = $db_filename;
        }
        if (!empty($db_directory) && !in_array($db_directory, $this->_paths['language_strings_db_directory'])) {
            $this->_paths['language_strings_db_directory'][] = $db_directory;
        }
        if (!empty($file) && !in_array($file, $this->_paths['loaded_files'])) {
            $this->_paths['loaded_files'][] = $file;
        }
        return $this;
    }