In return, you will get three lists. They contain the sockets that are actually
readable, writable and in error. Each of these lists is a subset (possibly
-empty) of the corresponding list you passed in. If you put a socket in more
-than one input list, it will only be (at most) in one output list.
+empty) of the corresponding list you passed in.
If a socket is in the output readable list, you can be
as-close-to-certain-as-we-ever-get-in-this-business that a ``recv`` on that