//package straightparse;

abstract class List<a> {
    public abstract <A> A accept (ListVisitor<A,a> v);
    public abstract int len();
    public abstract List<a> reverse ();
    public abstract List<a> reverse0 (List<a> acc);
    public abstract String toString();

}

class NIL<a> extends List<a> {
  public NIL () { }
  public int len() { return 0 ; } 
  public List<a> reverse () { return this; }
  public List<a> reverse0 (List<a> acc) { return acc; }
  public String toString() { return "" ; }
  public <A> A accept (ListVisitor<A,a> v) {
        return v.visit (this);
  }
}

class CONS<a> extends List<a> {
    public a hd;
    public List<a> tl;
    public CONS (a x, List<a> xs) {
        this.hd = x;
        this.tl = xs;
    }
    public int len() { return 1 + tl.len() ; } 
    public List<a> reverse () { return this.reverse0(new NIL<a>()) ;}
    public List<a> reverse0 (List<a> acc) { return this.tl.reverse0(new CONS<a>(this.hd,acc)) ;}
    public String toString() { if (this.tl instanceof NIL) {
                                 return this.hd.toString(); 
                               } else {
                                 return this.hd.toString() + "," + this.tl.toString(); 
			       } }

    public <A> A accept (ListVisitor<A,a> v) {
        return v.visit (this);
    }

}

interface ListVisitor<A,a> {
    public A visit (NIL<a> m);
    public A visit (CONS<a> m);
}