## Activity 4: Extend the specification of the service interface

1. Extend the specification in IService.interface with the remaining part of the signature according to the following informal description:

   - At any point in time, the vending machine may send an OutOfOrder notification (indicating some internal problem), going into error mode. Note that such an autonomous action of the server can be modelled as a transition without trigger and a notification in the "do"-part.

   - In error mode, the SwitchOn command fails and the commands to switch off and load product have no effect.

   - In error mode, the Reset command may bring the vending machine to the initial state where it is off. There is also a possibility that the Reset command fails. The Reset command is not allowed outside error mode.

2. Use monitoring to check the correctness of the IService specification with respect to the implementation:

   - To generate an OutOfOrder notification, click the CoinChecker Problem button of the coin checker interface.

   - After a Reset, select the result.