Function window_page 

async fn window_page<Window>(
    listen: &Listen<'_>,
    window: Window,
    from: PduCount,
    next_batch: u64,
) -> Option<Response>
where
    Window: Stream<Item = PdusIterItem> + Send,

The page for a window of new events, or None while the window is empty.

The peek keeps the event it looked at, so the page scans the window once.