sclass UpDownSequence is IntSize {
  BitBuffer moves; // up is true
  
  void add(UpDown move) {
    moves.add(move.bit);
  }
  
  toString { ret join(countIteratorAsList moveChar(size())); }
  
  S moveChar(int i) { ret moves.get(i) ? "U" : "D"; }
  
  public int size() { ret moves.size(); }
}