@Test
public void testAddToLeftCombiner() {

    final Set<String> abc = new HashSet<>(Arrays.asList("a", "b", "c"));
    final Set<String> oneTwoThree = new HashSet<>(Arrays.asList("1", "2", "3"));
    final BinaryOperator<Set<String>> operator = ChecklistExporter.addToLeft();
    final Set<String> result = operator.apply(abc, oneTwoThree);
    assertSame("result is same as left", abc, result);
    final Set<String> expected = new HashSet<>(Arrays.asList("a", "b", "c", "1", "2", "3"));
    assertEquals("result is expected", expected, result);
}