This example is a simple use of IF model-checking for SDL projects. The SDL
system is simply a server process creating a client process when receiving a
message. However, there should never be more than 2 clients running at a given
time. All request messages received while there are 2 clients running are
saved and treated only when one of the running clients dies.

This constraint is described in the IF observer 'obs' in the project. To avoid
the IF simulation to be too long, the observer restricts the number of request
messages sent to the server to 4. It also counts the number of clients
appearing and dying, making sure there are always 2 or less running. If more
than 2 clients are running at a time, it goes into the error state
nb_clients_error and stops.

This project contains the correct implementation of the server process, testing
the number of running clients as soon as they are created via a decision.
Checking the model should not report an error.
