// Introduced in Chapter 4
/** A last-in, first-out stack. */
public interface Stack<E> {

  /** Return true if this Stack is empty. */
  public boolean isEmpty();

  /**
   * Return the top item on this Stack, but do not modify the Stack.
   * @throws EmptyStructureException if this Stack is empty.
   */
  public E peek();

  /**
   * Remove and return the top item on this Stack.
   * @throws EmptyStructureException if this Stack is empty.
   */
  public E pop();

  /** Add targer to the top of this Stack. */
  public void push(E target);

}