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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 tss(self, up=0, down=0):
 """ Return a start, end tuple of positions around the transcription-start site Parameters up : int if greature than 0, the strand is used to add this many upstream bases in the appropriate direction down : int if greature than 0, the strand is used to add this many downstream bases into the gene. """

        if not self.is_gene_pred: return None
        tss = self.txEnd if self.strand == '-' else self.txStart
        start, end = tss, tss
        if self.strand == '+':
            start -= up
            end += down
        else:
            start += up
            end -= down
            start, end = end, start
        return max(0, start), max(end, start, 0)