public abstract long observeWm(int queueIndex, long wmValue);
@Test
    public void when_duplicateIdleMessage_then_processed() {
        // Duplicate idle messages are possible in this scenario:
        // A source instance emits IDLE_MESSAGE, then an event (not causing a WM) and then another
        // IDLE_MESSAGE again. The IDLE_MESSAGE is broadcast, but the event is not. So a downstream
        // instance can receive two IDLE_MESSAGE-s in a row.
        assertEquals(Long.MIN_VALUE, wc.observeWm(0, IDLE_MESSAGE.timestamp()));
        assertEquals(Long.MIN_VALUE, wc.observeWm(0, IDLE_MESSAGE.timestamp()));
    }