using System;
public class List {

  static int NULL=0;
  static int TOP=32767;

  List_Node hd;

  public int head ()       // head
  {
    if (hd != null)
      return (hd.getData());
    else {
      Console.WriteLine("*****ERROR***** hd applied to empty list");
      return TOP;
    }
  }

  public List tail ()       // tail
  {
    List Tail=new List();
    
    if (hd !=null && hd.tail !=null)
      Tail.hd=hd.tail;
    return Tail;
  }

  public List cons (int cont)     // cons
  {
    List newlist=new List();
    
    newlist.hd=new List_Node(hd, cont);
    return newlist;
  }

  public bool isNull ()
  {
    if (hd==null) 
      return true;
    else
      return false;
  } 
  
}

class List_Node 
{
  public List_Node tail;
  private int contents;
  
  public List_Node() 
  {
    tail=null;
    contents=0;
  }
  
  public List_Node(int cont)
  {
    tail=null;
    contents=cont;
  }

  public List_Node(List_Node theNextNode, int Cont)
  {
    tail=theNextNode;
    contents=Cont;
  }
  
  public int getData() 
  {
    return contents;
  }
}