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