Method rollback

src/main/java/org/sfj/PegLegParser.java:310–313

(int frameCount, SingleNode<V> oldTop)

  private int nextChar() { return source.nextChar(); }

  private void rollback(int frameCount, SingleNode<V> oldTop) {
    trimFramesTo(frameCount);
    restoreValues(oldTop);
  }

  public List<String> parseTrail() {
    List<String> ret = frame.stream().filter(Objects::nonNull).map(s -> s.name).collect(toList());