@Nonnull @Override
    public ProgressState call() {
        progTracker.reset();
        stateMachineStep();
        return progTracker.toProgressState();
    }
@Test
    public void when_item_then_offeredToSsWriter() {
        // When
        init(singletonList(entry("k", "v")));
        assertEquals(MADE_PROGRESS, sst.call());

        // Then
        assertEquals(entry(serialize("k"), serialize("v")), mockSsWriter.poll());
        assertNull(mockSsWriter.poll());
    }