An OTP application, though I don't know what that is. It's actually a way to use Zulip's API via Erlang and
here has been kindly allowing me to do that.
$ rebar3 compile
Run a shell via
rebar3 and peruse the available functions to your heart's content
$ rebar3 shell < ... Erlang shell stuff ... > message_loop(ZulipServer, Username, Password).
Obviously only use with a Zulip account whose private messages you won't miss, like a bot or on a development server.