// Data Definition: IntList := new Empty() + new Cons(int, IntList)

abstract class IntList {

  /** Returns String representation of this without enclosing parens. */
  abstract String toStringHelp();  
  
}

class Empty extends IntList {

  static Empty ONLY = new Empty(); // singleton pattern

  /* constructor */
  private Empty() {} 

  String toStringHelp() { return ""; }
  public String toString() { return "()"; }
}

class Cons extends IntList {

  int first;
  IntList rest;

  /* constructor */
  Cons(int f, IntList r) {
    first = f;
    rest = r;
  }

  /* accessors */
  int getFirst() { return first; }
  IntList getRest() { return rest;  }

  public String toString() {
    return "(" + first + rest.toStringHelp() + ")";
  }
  String toStringHelp() {
    return " " + first + rest.toStringHelp();  
  }
}