def called_alts_from_genotype(self):
        '''Returns a set of the (maybe REF and) ALT strings that were called, using GT in FORMAT.
        Returns None if GT not in the record'''
        if 'GT' not in self.FORMAT:
            return None

        genotype_indexes = set([int(x) for x in self.FORMAT['GT'].split('/')])
        alts = set()

        for i in genotype_indexes:
            if i == 0:
                alts.add(self.REF)
            else:
                alts.add(self.ALT[i-1])

        return alts
Returns a set of the (maybe REF and) ALT strings that were called, using GT in FORMAT.
        Returns None if GT not in the record