code: @Nonnull
    private static <E extends Comparable<? super E>> Set<E> ofSeqInternal(final Seq<E> seq) {
      if (seq.isEmpty()) {
        return empty();
      }
      final SeqSimpleSorted<E> seqSimpleSorted;
      if (seq instanceof SeqSimpleSorted) {
        seqSimpleSorted = (SeqSimpleSorted<E>) seq; // assumes it's distinct
      } else {
        seqSimpleSorted = (SeqSimpleSorted<E>) seq.sorted().distinct(); // if not a SeqSimpleSorted, make it one
      }
      return new Set<>(seqSimpleSorted);
    }
docs: INTERNAL: Assumes the seq is a sorted, distinct seq.