code: def pop_stream(cache, user_id):
      
      stack = cache.get(user_id)
      if stack is None:
          return None
  
      result = stack.pop()
  
      if len(stack) == 0:
          cache.delete(user_id)
      else:
          cache.set(user_id, stack)
  
      return result
docs: Pop an item off the stack in the cache. If stack
      is empty after pop, it deletes the stack.
  
      :param cache: werkzeug BasicCache-like object
      :param user_id: id of user, used as key in cache
  
      :return: top item from stack, otherwise None