persistable sclass UpDownSequence extends RandomAccessAbstractList<UpDown> {
  new BitBuffer moves; // up is true
  
  *(S s) {
    for (c : characters(upper(s)))
      if (c == 'U') add(UpDown.up);
      else if (c == 'D') add(UpDown.down);
  }
  
  public UpDown get(int i) {
    ret UpDown.fromBool(moves.get(i));
  }
  
  public bool add(UpDown move) {
    moves.add(move.bit);
    true;
  }
  
  toString { ret join(countIterator moveChar(size())); }
  
  S moveChar(int i) { ret moves.get(i) ? "U" : "D"; }
  
  public int size() { ret moves.size(); }
}