It is a basic example of terminal emulation with the "goat/term" package.
It reads chunks in and writes them to standard output. Try typing a line
and then hitting the up key on the next line. Try editing a previous line
and hitting the up key again.
Press ^C, ^D, or type "quit" to exit.
If something happens and you can't exit, try "killall goat" from another
terminal; this shouldn't happen, but it's possible.