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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def unchanged(self):
        '''
        Returns all keys that have been unchanged.

        If the keys are in child dictionaries they will be represented with
        . notation
        '''
        def _unchanged(current_dict, diffs, prefix):
            keys = []
            for key in current_dict.keys():
                if key not in diffs:
                    keys.append('{0}{1}'.format(prefix, key))
                elif isinstance(current_dict[key], dict):
                    if 'new' in diffs[key]:
                        # There is a diff
                        continue
                    else:
                        keys.extend(
                            _unchanged(current_dict[key],
                                       diffs[key],
                                       prefix='{0}{1}.'.format(prefix, key)))

            return keys
        return sorted(_unchanged(self.current_dict, self._diffs, prefix=''))