code: public function add(string $key, array $args = []) {
  		$this->data[self::TEXTS_KEY][$key] = array();
  		$this->data[self::PLACEHOLDER_JSON_KEY][$key] = $args;
  
  		foreach ($this->listeners as $listener) {
  			$listener->keyAdded($key, $this);
  		}
  	}
docs: Method adds key to {@see GroupData::$data} and triggers {@see GroupDataListener::keyAdded() listeners}.
  
  @param string $key