docs: static Matcher flattenNestedSequences(Matcher matcher) {
      if (matcher instanceof SeqMatcher) {
        List<Matcher> matchers = matcher.<SeqMatcher>as().matchers();
        List<Matcher> ms = new ArrayList<>();
        for (Matcher m : matchers) {
          if (m instanceof SeqMatcher) {
            ms.addAll(m.<SeqMatcher>as().matchers());
          } else {
            ms.add(m);
          }
        }
        return SeqMatcher.create(ms);
      }
      return matcher;
    }