code: def store_user_documents(user_document_gen, client, mongo_database_name, mongo_collection_name):
      
      mongo_database = client[mongo_database_name]
      mongo_collection = mongo_database[mongo_collection_name]
  
      # Iterate over all users to be annotated and store the Twitter lists in mongo.
      for user_twitter_id, user_document_list in user_document_gen:
          document = user_document_list
          document["_id"] = int(user_twitter_id)
          mongo_collection.update({"_id": user_twitter_id}, document, upsert=True)
docs: Stores Twitter list objects that a Twitter user is a member of in different mongo collections.
  
      Inputs: - user_document_gen: A python generator that yields a Twitter user id and an associated document list.
              - client: A pymongo MongoClient object.
              - mongo_database_name: The name of a Mongo database as a string.
              - mongo_collection_name: The name of the mongo collection as a string.