assertThat(fluent(1, 2, 3).toMap(Functions.toStringFunction()).entrySet())
        .containsExactly(immutableEntry(1, "1"), immutableEntry(2, "2"), immutableEntry(3, "3"))
        .inOrder();
  }

  public void testToMap_nullKey() {
    assertThrows(
        NullPointerException.class, () -> fluent(1, null, 2).toMap(Functions.constant("foo")));
  }

  public void testToMap_nullValue() {