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