/** Data Definition: IntList :=  new Empty() + new Cons(int, IntList) */
abstract class IntList {
}

class Empty extends IntList {

  int getFirst() { throw new IllegalArgumentException(
   "first requires a non Empty IntList");
  }
  IntList getRest() { throw new IllegalArgumentException(
   "rest requires a non Empty IntList");
  }

}

class Cons extends IntList {

  private int first;
  private IntList rest;

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

  /* accessors */

  /** returns first element of non-empty list */
  abstract int getFirst();    

  /** returns all but the first element of non-empty list */
  IntList getRest() { return rest; }
 
}