class ListOps
{

 private static List empty()
 {
  return LinkedList.empty();
 }

 public static List append(List l1,List l2)
 {
  if(l1.isempty())
     return l2;
  else
     {
      List a=append(l1.tail(),l2);
      return a.cons(l1.head());
     }
 }

 public static List merge(List l1,List l2)
 {
  if(l1.isempty())
     return l2;
  else if(l2.isempty())
     return l1;
  else
     {
      List m=merge(l1.tail(),l2.tail());
      return m.cons(l2.head()).cons(l1.head());
     }
 }

 public static List take(int n,List L)
 {
  if(n==0||L.isempty())
     return empty();
  else
     return take(n-1,L.tail()).cons(L.head());
 }

 public static List drop(int n,List L)
 {
  if(n==0||L.isempty())
     return L;
  else    
     return drop(n-1,L.tail());
 }

}