public void RemoveLineItem(PurchaseOrderLine line)
        {
            if (!_lineItems.Contains(line))
            {
                throw new PurchaseOrderTrackerException($"Line item is not part of this purchase order: {line}");
            }

            _lineItems.Remove(line);
        }
        public void AddLineItem(PurchaseOrderLine line)
        {
            if (!ProductIsFromSameSupplier(line.Product))
            {
                throw new PurchaseOrderTrackerException(
                          $"Line item product must be from the same supplier: {line.Product}");
            }

            _lineItems.Add(line);
        }