Believe it or not, I never programmed a TCP server or client before.
HTTP is so dominant now that, I guess, many people have no idea that
such things as Gopher ever existed. But for some reasons,
Isabelle/HOL has a pure TCP API, no HTTP included. So you either use
a native client or write your own. That wasn't particularly tricky,
but I still don't know how to unit-test that properly.
