public class RussianDoll {   // version 3

  private RussianDoll inner;
  private RussianDoll(RussianDoll rd) {  // let's make this private now
    inner = rd;
  }
  
  /* What's wrong with this?
   *   public static final RussianDoll ZERO = new RussianDoll(); */
  public static final RussianDoll ZERO = new RussianDoll(null);  

  public RussianDoll smaller() {
    return inner;   // but what is ZERO.smaller() now?
  }

  public RussianDoll bigger() {
    return new RussianDoll(this);
  }

  public String toString() {  // recursive: will it necessarily terminate?
    if (this == ZERO)
      return "0";
    else 
      return "S" + inner.toString();  // "S" for "surround"
  }

  public static void main(String args[]) {
    System.out.println(ZERO.bigger().bigger().bigger().smaller().bigger());
  }
}