class Cons extends List1
{
 private int first;
 private List1 next;

 public Cons(int h,List1 t)
 {
  first=h;
  next=t;
 }

 public int head()
 {
  return first;
 }

 public List1 tail()
 {
  return next;
 }

 public boolean isEmpty()
 {
  return false;
 }

 public String toString()
 {
  String str="["+first;
  for(List1 ptr=next; !ptr.isEmpty(); ptr=ptr.tail())
     str+=","+ptr.head();
  return str+"]";
 }
 
}