Function notify 

fn notify(
    notice: Notice,
    pressed: Option<Box<dyn FnOnce(&str) + Send>>,
) -> Result<(), String>

Shows notice. Without pressed, on the caller’s thread, the server’s refusal said. With it, on a thread of its own, which keeps the connection that showed it (the server tells a press to that connection: Plasma addresses it) and waits there (wait_for_press).