in folder s2, you can test the usage of SO_REUSEADDR option:
1) if you run python server.py first, and then python server.py, you will
   FAIL, because the address is already in use. This is the common case
2) if you run python server.pyt first, and then python server_another.py
   you will SUCCEED, as you specify the second server with SO_REUSEADDR
   option
3) if you run python server_another.py first, and then python server.py
   you will FAIL, as you must specify the second server with
   SO_REUSEADDR option, only specify the first one is useless