public function register(
    /*string*/ $id,
    callable $callback,
    /*int*/ $priority = self::PRIO_EARLY
  ) {
    // Normalize priority.
    $priority = intval($priority);

    // The priority must be valid.
    if(self::PRIO_START <= $priority && self::PRIO_END >= $priority) {
      // Register new callback.
      $this->registerImpl($id, $callback, $priority);
    }
  }