public static <K, V> WithKeys<K, V> of(SerializableFunction<V, K> fn) {
    checkNotNull(
        fn, "WithKeys constructed with null function. Did you mean WithKeys.of((Void) null)?");
    return new WithKeys<>(fn, null);
  }
@Test
  public void testWithKeysGetName() {
    assertEquals("WithKeys", WithKeys.<Integer, String>of(100).getName());
  }