Model Stack LIFO

boolean empty(): Tests if this stack is empty.
E   peek() : Looks at the object at the top of this stack without removing it from the stack.
E   pop() : Removes the object at the top of this stack and returns that object as the value of this function.
E   push(E item): Pushes an item onto the top of this stack.
int search(Object o): Returns the 1-based position where an object is on this stack.
