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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def count_documents(self, filter, session=None, **kwargs):
 """Count the number of documents in this collection. The :meth:`count_documents` method is supported in a transaction. All optional parameters should be passed as keyword arguments to this method. Valid options include: - `skip` (int):
 The number of matching documents to skip before returning results. - `limit` (int):
 The maximum number of documents to count. Must be a positive integer. If not provided, no limit is imposed. - `maxTimeMS` (int):
 The maximum amount of time to allow this operation to run, in milliseconds. - `collation` (optional):
 An instance of :class:`~pymongo.collation.Collation`. This option is only supported on MongoDB 3.4 and above. - `hint` (string or list of tuples):
 The index to use. Specify either the index name as a string or the index specification as a list of tuples (e.g. [('a', pymongo.ASCENDING), ('b', pymongo.ASCENDING)]). This option is only supported on MongoDB 3.6 and above. The :meth:`count_documents` method obeys the :attr:`read_preference` of this :class:`Collection`. .. note:: When migrating from :meth:`count` to :meth:`count_documents` the following query operators must be replaced: | Operator | Replacement | +=============+=====================================+ | $where | `$expr`_ | | $near | `$geoWithin`_ with `$center`_ | | $nearSphere | `$geoWithin`_ with `$centerSphere`_ | $expr requires MongoDB 3.6+ :Parameters: - `filter` (required):
 A query document that selects which documents to count in the collection. Can be an empty document to count all documents. - `session` (optional):
 a :class:`~pymongo.client_session.ClientSession`. - `**kwargs` (optional):
 See list of options above. .. versionadded:: 3.7 .. _$expr: https://docs.mongodb.com/manual/reference/operator/query/expr/ .. _$geoWithin: https://docs.mongodb.com/manual/reference/operator/query/geoWithin/ .. _$center: https://docs.mongodb.com/manual/reference/operator/query/center/#op._S_center .. _$centerSphere: https://docs.mongodb.com/manual/reference/operator/query/centerSphere/#op._S_centerSphere """

        pipeline = [{'$match': filter}]
        if 'skip' in kwargs:
            pipeline.append({'$skip': kwargs.pop('skip')})
        if 'limit' in kwargs:
            pipeline.append({'$limit': kwargs.pop('limit')})
        pipeline.append({'$group': {'_id': 1, 'n': {'$sum': 1}}})
        cmd = SON([('aggregate', self.__name),
                   ('pipeline', pipeline),
                   ('cursor', {})])
        if "hint" in kwargs and not isinstance(kwargs["hint"], string_type):
            kwargs["hint"] = helpers._index_document(kwargs["hint"])
        collation = validate_collation_or_none(kwargs.pop('collation', None))
        cmd.update(kwargs)

        def _cmd(session, server, sock_info, slave_ok):
            result = self._aggregate_one_result(
                sock_info, slave_ok, cmd, collation, session)
            if not result:
                return 0
            return result['n']

        return self.__database.client._retryable_read(
            _cmd, self._read_preference_for(session), session)