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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 _step99(self, in_row, tmp_row, out_row):
 """ Validates all mandatory fields are in the output row and are filled. :param dict in_row: The input row. :param dict tmp_row: Not used. :param dict out_row: The output row. """

        park_info = ''
        for field in self._mandatory_fields:
            if field not in out_row or not out_row[field]:
                if park_info:
                    park_info += ' '
                park_info += field

        return park_info, None