code: def add_alias(self, alias, index):
          
          if index >= len(self._datasets):
              raise DataInvalidIndex('A dataset with index {} does not exist'.format(index))
          self._aliases[alias] = index
docs: Add an alias pointing to the specified index.
  
          Args:
              alias (str): The alias that should point to the given index.
              index (int): The index of the dataset for which an alias should be added.
  
          Raises:
              DataInvalidIndex: If the index does not represent a valid dataset.