The program must manage the bookings of a hairdresser shop.
A worker can do two different services at the same time (eg Schampo and Hair cut) according to customer requests. The system does not prevent double booking at the same time and this is an ERROR.
If a customer books me a haircut with Lucia at 9.30 another customer cannot book a SHAMPO at 9.30 always with Lucia because Lucia can only do one service at a time even if she is enabled for both. The principle is that the first one who arrives is served for one of the two services that Lucia can do and the other must choose a different time that is still free.