Where is created $class varicle in presenterFactory.php?

Hi, here in PresenterFactory is on line 36 varible $class and i still can't understand, where is filled.

	/**
	 * @param  callable  function (string $class): IPresenter
	 */
	public function __construct(callable $factory = NULL)
	{
		$this->factory = $factory ?: function ($class) { return new $class; };
	}

It's bug or any class property? Thanks.