@Contract(pure = true)
  public static @Unmodifiable @NotNull <T> List<T> createMaybeSingletonList(@Nullable T element) {
    //noinspection SSBasedInspection
    return element == null ? Collections.emptyList() : Collections.singletonList(element);
  }
@Contract(pure = true)
  public static @Unmodifiable @NotNull <T> List<T> createMaybeSingletonList(@Nullable T element)