/**
  * add product
  * 
  * @param tpyProductInterface $product
  * @param int                     $count
  * 
  * @return tpyCartInterface
  */
 public function addProduct(tpyProductInterface $product, $count = 1)
 {
     foreach ($this->getItems() as $item) {
         if ($item->getProductIdentifier() == $product->getUid()) {
             $cartItem = $item;
             break;
         }
     }
     if (isset($cartItem)) {
         $cartItem->increaseCount($count);
     } else {
         $cartItem = new tpyCartItem();
         $cartItem->setCount($count)->setProductIdentifier($product->getUid())->setProductData($product->toJson());
         $this->getItems()->add($cartItem);
     }
     return $this;
 }