Solution $4\mathrm{.}6$ A restaurant chain submits a purchase order $($PO$)$ to replenish it warehouses every Thursday. The restaurant chain$$s procurement system assumes the receipt of either a $$PO Response$$ or an error message. However, it may also happen that no response is received at all due to system errors or due to delays in handling the PO on the supplier$$s side. If no response is received by Friday afternoon or if an error message is received, a purchasing officer at the restaurant chain$$s headquarters should be notified. Otherwise, the PO Response is processed normally.