class CharListD implements CharList
{
 private InnerList content;

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

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

 public CharList tail()
 {
  return new CharListD(content.next);
 }

 public CharList cons(char ch)
 {
  return new CharListD(new InnerList(ch,content));
 }

 public static CharList empty()
 {
  return new CharListD(null);
 }

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

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