code: @Beta
    public static LongStream stream(OptionalLong optional) {
      return optional.isPresent() ? LongStream.of(optional.getAsLong()) : LongStream.empty();
    }
docs: If a value is present in {@code optional}, returns a stream containing only that element,
  otherwise returns an empty stream.
  
  <p><b>Java 9 users:</b> use {@code optional.stream()} instead.