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)