额，输出调试信息后发现 tmp 和 stack[-1] 的地址竟然是一样的，为甚么啊

from sys import stdin

stack = []
t = int(input())
for _ in range(t):
    n = int(input())
    for __ in range(n):
        cmd = stdin.readline()
        if cmd == "PUSH\n":
            stack.append([])
        elif cmd == "DUP\n":
            stack.append(stack[-1])
        elif cmd == "UNION\n":
            tmp = stack.pop()
            stack[-1] += [i for i in tmp if i not in stack[-1]]
        elif cmd == "INTERSECT\n":
            tmp = stack.pop()
            stack[-1] = [i for i in stack[-1] if i in tmp]
        else: # ADD 出错了，相当于做了 x.append(x) 的事
            tmp = stack.pop()
            print(id(tmp), id(stack[-1])) # 明明 len(stack) = 2，为甚么这两个地址一样啊
            # 难道 pop() 有问题？
            if (tmp not in stack[-1]): stack[-1].append(tmp)
        print(len(stack[-1]))
    print("***")