code: def _get_unique_seqid(self, alias, namespace):
          
  
          recs = self.aliases.find_aliases(alias=alias, namespace=namespace)
          seq_ids = set(r["seq_id"] for r in recs)
          if len(seq_ids) == 0:
              raise KeyError("Alias {} (namespace: {})".format(alias, namespace))
          if len(seq_ids) > 1:
              # This should only happen when namespace is None
              raise KeyError("Alias {} (namespace: {}): not unique".format(alias, namespace))
          return seq_ids.pop()
docs: given alias and namespace, return seq_id if exactly one distinct
          sequence id is found, raise KeyError if there's no match, or
          raise ValueError if there's more than one match.