public void testGetOnlyElement_withDefault_empty_null() {
    Iterable<String> iterable = emptyList();
    assertNull(Iterables.<@Nullable String>getOnlyElement(iterable, null));
  }

  public void testGetOnlyElement_withDefault_multiple() {
    Iterable<String> iterable = asList("foo", "bar");
    assertThrows(IllegalArgumentException.class, () -> getOnlyElement(iterable, "x"));
  }