def _fetch_AlignmentMapper(self, tx_ac, alt_ac=None, alt_aln_method=None):
        """convenience version of VariantMapper._fetch_AlignmentMapper that
        derives alt_ac from transcript, assembly, and alt_aln_method
        used to instantiate the AssemblyMapper instance

        """

        if alt_ac is None:
            alt_ac = self._alt_ac_for_tx_ac(tx_ac)
        if alt_aln_method is None:
            alt_aln_method = self.alt_aln_method
        return super(AssemblyMapper, self)._fetch_AlignmentMapper(tx_ac, alt_ac, alt_aln_method)
convenience version of VariantMapper._fetch_AlignmentMapper that
        derives alt_ac from transcript, assembly, and alt_aln_method
        used to instantiate the AssemblyMapper instance