def _mutated_contents(self):
raise NotImplementedError(<EOL>"<STR_LIT>".format(type(self))<EOL>)<EOL>
The mutated contents of an info.

        When an info is asked to mutate, this method will be executed
        in order to determine the contents of the new info created.

        The base class function raises an error and so must be overwritten
        to be used.