code: def top_stream(cache, user_id):
      
      if not user_id:
          return None
      
      stack = cache.get(user_id)
      if stack is None:
          return None
      return stack.pop()
docs: Peek at the top of the stack in the cache.
  
      :param cache: werkzeug BasicCache-like object
      :param user_id: id of user, used as key in cache
  
      :return: top item in user's cached stack, otherwise None