public abstract long observeWm(int queueIndex, long wmValue);
@Test
    public void when_wmGoesBack_then_error() {
        assertEquals(Long.MIN_VALUE, wc.observeWm(0, 10));

        assertThatThrownBy(() -> wc.observeWm(0, 9))
                .hasMessageContaining("not monotonically increasing");
    }