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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 _get_fout_go(self):
 """Get the name of an output file based on the top GO term."""

        assert self.goids, "NO VALID GO IDs WERE PROVIDED AS STARTING POINTS FOR HIERARCHY REPORT"
        base = next(iter(self.goids)).replace(':', '')
        upstr = '_up' if 'up' in self.kws else ''
        return "hier_{BASE}{UP}.{EXT}".format(BASE=base, UP=upstr, EXT='txt')