public static void main(String[] args) {
   Snake s = new Snake(5, 'a');
   System.out.println("s = " + s);
   Snake s2 = (Snake) s.clone();
   System.out.println("s2 = " + s2);
   s.increment();
   System.out.println("after s.increment, s2 = " + s2);
   monitor.expect(
       new String[] {"s = :a:b:c:d:e", "s2 = :a:b:c:d:e", "after s.increment, s2 = :a:c:d:e:f"});
 }
 public String toString() {
   String s = ":" + c;
   if (next != null) s += next.toString();
   return s;
 }
 public void increment() {
   c++;
   if (next != null) next.increment();
 }