In CMU CL, while a client socket is simple-socket-stream, a socket
returned by socket-accept is created with
(make-fd-stream (accept-tcp-connection , and this is an fd-stream.
The simplest way to handle it is to declare type socket as stream,
and not simple-socket-stream, in (or cmu scl) (I am not sure about
scl, but there is no trial download there).