@Override
    public String toString() {
      StringBuilder sb = new StringBuilder();
      for (int i = 0, limit = Math.min(myFrom.size(), myNext.size()); i < limit; i++) {
        if (sb.length() != 0) sb.append(", ");
        sb.append(myFrom.get(i)).append("->").append(myNext.get(i));
      }
      return sb.toString();
    }
@Override
    public String toString()