/**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code
   * ImmutableSet}. Elements appear in the resulting set in the encounter order of the stream; if
   * the stream contains duplicates (according to {@link Object#equals(Object)}), only the first
   * duplicate in encounter order will appear in the result.
   *
   * @since 33.2.0 (available since 21.0 in guava-jre)
   */