code: func (c *Corpus) signerID(signer blob.Ref) string {
  	if !signer.Valid() {
  		return ""
  	}
  	id, ok := c.keyId[signer]
  	if !ok {
  		return "unknown GPG key"
  	}
  	return id
  }
docs: // signerID returns the GPG ID (e.g. 2931A67C26F5ABDA) corresponding to the
  // signer. It returns the empty value if signer is not a valid blob Ref, and the
  // unusable value "unknown GPG key" when it applies, as a convenience to help
  // with the different cases covered by valuesAtSigner.