def letter_queue(commands):

queue = []

for command in commands:

if command == 'POP':

if len(queue) > 0:

queue.pop(0)

else:

queue.append(command.split(' ')[1])

return ''.join(queue)

if __name__ == '__main__':

# These "asserts" using only for self-checking and not necessary for auto-testing

assert letter_queue(("PUSH A", "POP", "POP", "PUSH Z", "PUSH D", "PUSH O", "POP", "PUSH T")) == "DOT", "dot example"

assert letter_queue(("POP", "POP")) == "", "Pop, Pop, empty"

assert letter_queue(("PUSH H", "PUSH I")) == "HI", "Hi!"

assert letter_queue(()) == "", "Nothing"

print("All done? Earn rewards by using the 'Check' button!")