class LinkedList implements List
{
 private Cell myList;

 LinkedList()
 {
  myList=null;
 }

 private LinkedList(Cell list)
 {
  myList=list;
 }

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

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

 public List tail()
 {
  return new LinkedList(myList.rest);
 }

 public List cons(Object obj)
 {
  return new LinkedList(new Cell(obj,myList));
 }

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

 public String toString()
 {
  if(isempty())
     return "[]";
  else
     return "["+head()+restToString(tail());
 }

 private static String restToString(List L)
 {
  if(L.isempty())
     return "]";
  else
     return ","+L.head()+restToString(L.tail());
 }

 private static class Cell
 {
  Object first;
  Cell rest;

  Cell(Object h,Cell t)
  {
   first=h;
   rest=t;
  }
 }
 
}