import java.util.*;

abstract class List1
{

 public static List1 empty()
 {
  return new Empty();
 }

 public abstract int head();

 public abstract List1 tail();

 public List1 cons(int n)
 {
  return new Cons(n,this);
 }

 public abstract boolean isEmpty();
 
 public static List1 fromString(String str)
 {
  str=str.trim();
  if(str.charAt(0)!='['||str.charAt(str.length()-1)!=']')
     return new Empty();
  str = str.substring(1,str.length()-1);
  StringTokenizer toks = new StringTokenizer(str,",");
  try {
       return make(toks);
      }
  catch(NumberFormatException e)
      {
       return new Empty();
      }
 }

 private static List1 make(StringTokenizer toks)
 {
  if(toks.hasMoreTokens())
     {
      String num = toks.nextToken().trim();
      return new Cons(Integer.parseInt(num),make(toks));
     }
  else
     return new Empty();
 }

}