Traceback (most recent call last):                                                                                                                                                       
  File "labs/aggregations/ch3_1.py", line 30, in <module>                                                                                                                            
    cur = col.aggregate(pipeline)                                                                                                                                                       
  File "/home/extraymond/.pyenv/versions/umongo/lib/python3.7/site-packages/pymongo/collection.py", line 2397, in aggregate                                                               
    **kwargs)                                                                                       
  File "/home/extraymond/.pyenv/versions/umongo/lib/python3.7/site-packages/pymongo/collection.py", line 2304, in _aggregate                                                          
    client=self.__database.client)                                                                                                                                   
  File "/home/extraymond/.pyenv/versions/umongo/lib/python3.7/site-packages/pymongo/pool.py", line 579, in command                                                                   
    unacknowledged=unacknowledged)                                                                                                                                                     
  File "/home/extraymond/.pyenv/versions/umongo/lib/python3.7/site-packages/pymongo/network.py", line 150, in command                                                                  
    parse_write_concern_error=parse_write_concern_error)                                                                                                                               
  File "/home/extraymond/.pyenv/versions/umongo/lib/python3.7/site-packages/pymongo/helpers.py", line 155, in _check_command_response                                                
    raise OperationFailure(msg % errmsg, code, response)                                                                                                                                
pymongo.errors.OperationFailure: Unrecognized expression '$toInt'