docs: def _validate_outgroup(self, outgroup):
          """All voucher codes in our datasets have dashes converted to underscores."""
          if outgroup:
              outgroup = outgroup.replace("-", "_")
              good_outgroup = False
              for seq_record in self.seq_records:
                  if seq_record.voucher_code == outgroup:
                      good_outgroup = True
                      break
              if good_outgroup:
                  self.outgroup = outgroup
              else:
                  raise ValueError("The given outgroup {0!r} cannot be found in the "
                                   "input sequence records.".format(outgroup))
          else:
              self.outgroup = None