public class Stack {

    public class Node {
	Symbol s;
	Node next;
	Symbol getSymbol(){return s;}
	Node getNext(){return next;}
	Node(Symbol ps, Node nl) {
	    s = ps;
	    next = nl;
	}
    }

    Node head;

    public Stack() {
	head = null; 
    }

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

    public void push(Symbol s) 
    {
	head = new Node(s, head);
    }

    // requires !this.isEmpty() 
    public Symbol pop()
    {
	Symbol s;
	if (head != null) {
	    s = head.getSymbol();
	    head = head.getNext();
	    return s;
	} else return null;
    }

    // requires !this.isEmpty() 
    public Symbol top() 
    {
	Symbol s;
	if (head != null) {
	    s = head.getSymbol();
	    return s;
	} else return null;
    }

}