// una semplice implementazione delle pile
public class Pila{
    private L list;

    public Pila (){
        list = null;
    }

    public void push(int x){
        list = new L(x, list);
    }

    public int top(){
        if (list == null){
            throw new IllegalAccessError("pila vuota");
        }
        else{
            return list.head();
        }
    }

    public void pop(){
        if (list == null){
               throw new IllegalAccessError("pila vuota");
           }
           else{
               list = list.tail();
           }
    }

    public boolean isEmpty(){
        if (list == null)
            return true;
        else return false;
    }

//    public String toString(){
//        if (list == null){
//            return "[]";
//        }
//        else{
//            return list.toString();
//        }
//    }

    private String stp(L l){
        if (l == null) return "      --";
        else return "      "+l.head() + "\n" +stp(l.tail());
    }
    public String toString(){
        return stp(list);
    }
}