public function productByOrderItemId($item_id, \WC_Abstract_Order $WC_Order = null)
    {
        // WC 3.0+ uses `\WC_Order_Item_Product` extending `\WC_Order_Item`.
        // <https://docs.woocommerce.com/wc-apidocs/class-WC_Order_Item_Product.html>
        // <https://docs.woocommerce.com/wc-apidocs/class-WC_Order_Item.html>

        if (!($item_id = (int) $item_id)) {
            return null; // Not possible.
        } elseif (!($WC_Order = $WC_Order ?: $this->orderByItemId($item_id))) {
            return null; // Not possible.
        }
        foreach ($WC_Order->get_items() as $_item_id => $_WC_Order_Item) {
            if ($_item_id === $item_id) {
                if ($_WC_Order_Item instanceof \WC_Order_Item_Product) {
                    $WC_Product = $_WC_Order_Item->get_product();
                    return $WC_Product instanceof \WC_Product && $WC_Product->exists() ? $WC_Product : null;
                    //
                } elseif ($_WC_Order_Item instanceof \WC_Order_Item) {
                    return null; // Not a product item.
                    //
                } else { // Back compat.
                    $_item      = (array) $_WC_Order_Item;
                    $WC_Product = $WC_Order->get_product_from_item($_item);
                    return $WC_Product instanceof \WC_Product && $WC_Product->exists() ? $WC_Product : null;
                }
            }
        } // unset($_item_id, $_WC_Order_Item, $_item); // Housekeeping.

        return null; // Failure.
    }