public interface StackInterface {
  public boolean isEmpty();
  // Determines whether the stack is empty.
  // Precondition: None.
  // Postcondition: Returns true if the stack is empty;
  // otherwise returns false.

  public void popAll();
  // Removes all the items from the stack.
  // Precondition: None.
  // PostCondition: Stack is empty.

  public void push(Object newItem) throws StackException;
  // Adds an item to the top of a stack.
  // Precondition: newItem is the item to be added.
  // Postcondition: If insertion is successful, newItem
  // is on the top of the stack.
  // Exception: Some implementations may throw 
  // StackException when newItem cannot be placed on
  // the stack.

  public Object pop() throws StackException;
  // Removes the top of a stack.
  // Precondition: None.
  // Postcondition: If the stack is not empty, the item 
  // that was added most recently is removed from the
  // stack and returned.
  // Exception: Throws StackException if the stack is 
  // empty.
  
  public Object peek() throws StackException;
  // Retrieves the top of a stack.
  // Precondition: None.
  // Postcondition: If the stack is not empty, the item
  // that was added most recently is returned. The 
  // stack is unchanged. 
  // Exception: Throws StackException if the stack is 
  // empty.
}  // end StackInterface