class CharListE implements CharList
{
// A CharList implementation using Strings

 private String content;

 private CharListE(String L)
 {
  content=L;
 }

 public char head()
 {
  return content.charAt(0);
 }

 public CharList tail()
 {
  int n=content.length()
  return new CharListE(content.substring(1,n));
 }

 public CharList cons(char ch)
 {
  return new CharListE(ch+content);
 }

 public static CharList empty()
 {
  return new CharListE("");
 }

 public boolean isempty()
 {
  return (content.length()==0);
 }

}