java8.java

  public Stream<Verse> allVerses() {
    return books().stream().reduce(
        Stream.empty(),
        (sa, b) -> b.chapters().stream().reduce(
            sa,
            (sb, ch) -> Stream.concat(sb, ch.verses().stream()),
            (sb, sc) -> Stream.concat(sb, sc)),
        (sb, sc) -> Stream.concat(sb, sc));
  }