abstract class IntSList {

  // toStringHelp: return a string version of the list
  // which contains no parens, just comma-separated items:
  //
  abstract protected String toStringHelp();

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

  // main is a great place to put test cases:
  //
  public static void main( String[] args ) {
    IntSList l0, l1, l2, l3;

    l0 = new Empty();
    l1 = new Cons( 99, l0 );
    l2 = new Cons( 5, new Cons ( 6, new Empty() ));
    l3 = new Cons( 4, l2 );

    System.out.println( "l0 is " + l0.toString() );
    System.out.println( "l1 is " + l1.toString() );
    System.out.println( "l2 is " + l2.toString() );
    System.out.println( "l3 is " + l3.toString() );
    }
  }



// ------------ Empty --------------
//
class Empty extends IntSList {

  Empty() {}

  protected String toStringHelp() {
    return "";
    }
  }


// ------------ Cons ---------------
//
class Cons extends IntSList {
  int first;
  IntSList rest;

  Cons( int car, IntSList cdr ) {
    first = car;
    rest  = cdr;
    }

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

  }