protected function isTrueForBook(Book $book)
 {
     if ($this->isEasy()) {
         return $book->isEasy();
     }
     if ($this->isNormal()) {
         return $book->isNormal();
     }
     if ($this->isHard()) {
         return $book->isHard();
     }
 }
 protected function isTrueForBook(Book $book)
 {
     return mb_strlen($book->getTitle()) > $this->threshold;
 }