@Contract(pure = true)
  public static @Unmodifiable @NotNull <T> Set<T> createMaybeSingletonSet(@Nullable T element) {
    return element == null ? Collections.emptySet() : Collections.singleton(element);
  }
@Contract(pure = true)
  public static @Unmodifiable @NotNull <T> Set<T> createMaybeSingletonSet(@Nullable T element)