Returns true iff there is a sequence called self.CHROM in the
        dict of ref_sequences, and the REF string matches

def ref_string_matches_dict_of_ref_sequences(self, ref_sequences):
        '''Returns true iff there is a sequence called self.CHROM in the
        dict of ref_sequences, and the REF string matches'''
        return self.CHROM in ref_sequences and self.ref_string_matches_ref_sequence(ref_sequences[self.CHROM])