/** Stack ADT */
public interface Stack<E> {
    /** Reinitialize the stack. The user is responsible for reclaiming the storage used by the stack elements. */
    public void clear();

    /** Push an element ontp the top of the stack.
	@param it The element being pushed onto the stack. */
    public void push(E it);

    /** Remove and return the element at the top of the stack.
	@return The element at the top of the stack. */
    public E pop();

    /** @return A copy of the top element. */
    public E topValue();

    /** @return The number of elements in the stack. */
    public int length();
};