nodes: 0, module; 1, function_definition; 2, function_name:retrieve; 3, parameters; 4, block; 5, identifier:self; 6, identifier:operation; 7, default_parameter; 8, expression_statement; 9, expression_statement; 10, if_statement; 11, return_statement; 12, identifier:field; 13, None; 14, comment:"""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
          """; 15, assignment; 16, comparison_operator:obj is None; 17, comment:# empty Mark instance; 18, block; 19, call; 20, identifier:obj; 21, call; 22, identifier:obj; 23, None; 24, return_statement; 25, attribute; 26, argument_list; 27, attribute; 28, argument_list; 29, call; 30, identifier:Mark; 31, identifier:from_dict; 32, attribute; 33, identifier:obj; 34, identifier:self; 35, identifier:_get; 36, identifier:operation; 37, identifier:field; 38, identifier:Mark; 39, argument_list; 40, identifier:self; 41, identifier:collection; 42, keyword_argument; 43, keyword_argument; 44, keyword_argument; 45, identifier:collection; 46, attribute; 47, identifier:operation; 48, identifier:operation; 49, identifier:field; 50, identifier:field; 51, identifier:self; 52, identifier:collection
code: 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)