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

  public void testGetOnlyElement_withDefault_singleton() {
    Iterable<String> iterable = singletonList("foo");
    assertThat(getOnlyElement(iterable, "bar")).isEqualTo("foo");
  }

  public void testGetOnlyElement_withDefault_empty() {