public function whenBookOfferRetracted(BookOfferRetracted $event)
 {
     $this->books->remove($event->bookId());
 }
 /** @test */
 public function an_event_holds_data()
 {
     $bookId = BookId::generate();
     $event = new BookOfferRetracted($bookId);
     $this->assertEquals($event->bookId(), $bookId);
 }