from collections import deque
class Queue:
    def __init__(self): self.items = deque()
    def enqueue(self, value): self.items.append(value)
    def dequeue(self):
        if not self.items: raise IndexError("queue underflow")
        return self.items.popleft()
    def is_empty(self): return not self.items

q = Queue(); q.enqueue('A'); q.enqueue('B')
assert q.dequeue() == 'A' and q.dequeue() == 'B' and q.is_empty()
print('Queue tests passed')