class LinkedList implements List
{
 private InnerList content;

 private LinkedList(InnerList L)
 {
  content=L;
 }

 public Object head()
 {
  return content.first;
 }

 public List tail()
 {
  return new LinkedList(content.next);
 }

 public List cons(Object h)
 {
  return new LinkedList(new InnerList(h,content));
 }

 public static List empty()
 {
  return new LinkedList(null);
 }

 public boolean isempty()
 {
  return (content==null);
 }

 private static class InnerList
 {
  Object first;
  InnerList next;
 
  InnerList(Object hd,InnerList tl)
  {
   first=hd;
   next=tl;
  }
 }
    
}