return new HashSet<>();
  }

  /**
   * Creates a <i>mutable</i> {@code HashSet} instance initially containing the given elements.
   *
   * <p><b>Note:</b> if elements are non-null and won't be added or removed after this point, use
   * {@link ImmutableSet#of()} or {@link ImmutableSet#copyOf(Object[])} instead. If {@code E} is an
   * {@link Enum} type, use {@link EnumSet#of(Enum, Enum[])} instead. Otherwise, strongly consider