pub fn chat_server<'a, P>(
process: &Process<'a, P>,
in_stream: KeyedStream<u64, String, Process<'a, P>, Unbounded>,
membership: KeyedStream<u64, MembershipEvent, Process<'a, P>, Unbounded>,
nondet_user_arrival_broadcast: NonDet,
) -> KeyedStream<u64, String, Process<'a, P>, Unbounded, NoOrder>