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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 _get(self, operation, field):
 """Get tracked position for a given operation and field."""

        self._check_exists()
        query = {Mark.FLD_OP: operation.name,
                 Mark.FLD_MARK + "." + field: {"$exists": True}}
        return self._track.find_one(query)