import java.util.*;

class List
{
 private Cell llist;

 public static List empty()
 {
  return new List(null);
 }

 private List(Cell l)
 {
  llist=l;
 }

 public int head()
 {
  return llist.first;
 }

 public List tail()
 {
  return new List(llist.next);
 }

 public List cons(int n)
 {
  return new List(new Cell(n, llist));
 }

 public boolean isEmpty()
 {
  return llist==null;
 }

 public String toString()
 {
  if(llist==null)
     return "[]";
  else
     {
      String str="["+llist.first;
      for(Cell ptr=llist.next; ptr!=null; ptr=ptr.next)
         str+=","+ptr.first;
      return str+"]";
     }
 }
 
 public static List fromString(String str)
 {
  str=str.trim();
  if(str.charAt(0)!='['||str.charAt(str.length()-1)!=']')
     return empty();
  str = str.substring(1,str.length()-1);
  StringTokenizer toks = new StringTokenizer(str,",");
  try {
       return new List(make(toks));
      }
  catch(NumberFormatException e)
      {
       return empty();
      }
 }

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

 private static class Cell
 {
  int first;
  Cell next;

  Cell(int f,Cell n)
  {
   first=f;
   next=n;
  }
 }
}