code: def retrieve(self, operation, field=None):
          
          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)
docs: 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