public void testGetOnlyElement_noDefault_empty() {
    Iterable<String> iterable = emptyList();
    assertThrows(NoSuchElementException.class, () -> getOnlyElement(iterable));
  }

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

  public void testGetOnlyElement_withDefault_singleton() {