code: func (x *Index) signerRefs(ctx context.Context, keyID string) (SignerRefSet, error) {
  	if x.corpus != nil {
  		return x.corpus.signerRefs[keyID], nil
  	}
  	it := x.queryPrefixString(keySignerKeyID.name)
  	var err error
  	var refs SignerRefSet
  	defer closeIterator(it, &err)
  	prefix := keySignerKeyID.name + ":"
  	for it.Next() {
  		if it.Value() == keyID {
  			refs = append(refs, strings.TrimPrefix(it.Key(), prefix))
  		}
  	}
  	return refs, nil
  }
docs: // signerRefs returns the set of signer blobRefs matching the signer keyID. It
  // does not return an error if none is found.