public Integer apply(String input) {
                          return input.length();
                        }
                      });
        });
  }

  public void testUniqueIndex_nullKey() {
    assertThrows(
        NullPointerException.class, () -> fluent(1, 2, 3).uniqueIndex(Functions.constant(null)));
  }

  public void testUniqueIndex_nullValue() {
    assertThrows(