What does the prison allow its prisoners to do? It allows its prisoners to ____

There is a prison in Iceland which allows its prisoners to go out without any guards to watch every day. They work on the farm near the prison during the day，and come back to have their evening meal and to sleep every evening. Before they are allowed to go out like this, they have to promise to come back every evening. If they do not promise this, they are not let out.
    One night one of the prisoners was invited to have a meal and a drink with the family of the farmer he was working for, so he came back to the prison very late. He had to knock at the gate several times before the guards came to let him in. The guard did not like being disturbed at this time, so he said to the prisoner angrily, "If you come back so late again, I won’t let you in."