sclass UniqueString implements CharSequence {
  S s;
  
  private *() {}
  *(S *s, bool dummy) {} // avoid accidental calling
  
  toString { ret s; }
  
  public char charAt(int index) { ret s.charAt(index); }
  public int length() { ret s.length(); }
  public CharSequence subSequence(int start, int end) {
    ret uniqueString(s.substring(start, end));
  }
}