class Stack:
    def __init__(self): self.items = []
    def push(self, value): self.items.append(value)
    def pop(self):
        if not self.items: raise IndexError("stack underflow")
        return self.items.pop()
    def peek(self):
        if not self.items: raise IndexError("stack empty")
        return self.items[-1]

s = Stack()
s.push('A'); s.push('B')
assert s.peek() == 'B' and s.pop() == 'B' and s.pop() == 'A'
print('Stack tests passed')