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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 retrieve(self, operation, field=None):
 """Retrieve a position in this collection. :param operation: Name of an operation :type operation: :class:`Operation` :param field: Name of field for sort order :type field: str :return: The position for this operation :rtype: Mark :raises: NoTrackingCollection """

        obj = self._get(operation, field)
        if obj is None:
            # empty Mark instance
            return Mark(collection=self.collection, operation=operation, field=field)
        return Mark.from_dict(self.collection, obj)