Protocol-complete Stack

Implement class Stack that supports: - push(x), pop() (raise IndexError on empty), peek() (raise IndexError on empty) - len(stack), stack[i] (int indexing, negative included), x in stack, iteration (bottom→top) - slicing: stack[1:3] returns a NEW Stack - __eq__ against another Stack comparing element order (return NotImplemented for non-Stack) - __repr__ as Stack(<items joined by ', '>)

Difficulty: advanced