class Li<A>{
    A hd=null;
    Li<A> tl=null;
    Li(){}
    Li(A hd,Li<A> tl){this.hd=hd;this.tl=tl;}
    boolean isEmpty(){return hd==null&tl==null;}
    @Override public String toString(){
	if (isEmpty()) return "[]"; 
	return "["+hd+","+tl+"]";
    }
}