Library

In a university in N city, the normal dormitories are all four-person dormitories. If a class is not full, multiple classes will be mixed, but there is only one dormitory named 513 with only three people. When several members inside asked about the teacher, the teacher's answer was that there were always four people in their dormitory, and these three people had to abide by many inexplicable rules wherever they went. What was hidden behind all this...