public boolean unregisterOutput(String id) {
        final PartialWatermark output = watermarkPerOutputId.remove(id);
        if (output != null) {
            combinedWatermarkStatus.remove(output);
            return true;
        } else {
            return false;
        }
    }
@Test
    void testRemoveNotRegisteredReturnValue() {
        final TestingWatermarkOutput underlyingWatermarkOutput = createTestingWatermarkOutput();
        final WatermarkOutputMultiplexer multiplexer =
                new WatermarkOutputMultiplexer(underlyingWatermarkOutput);

        final boolean unregistered = multiplexer.unregisterOutput("does-not-exist");

        assertThat(unregistered).isFalse();
    }