/**
  * @param Product $product
  */
 public function addProduct(Product $product)
 {
     if (!isset($this->products[$product->title()])) {
         $this->products[$product->title()] = $product;
     }
 }
 /**
  * @param Product $product
  *
  * @return bool
  */
 private function productExists(Product $product)
 {
     return $this->findBy(['title' => $product->title()]);
 }