code: func ParseSerialNumber(serial, aki string, dbAccessor certdb.Accessor) (*Certificate, error) {
  	normalizedAKI := strings.ToLower(aki)
  	normalizedAKI = strings.Replace(normalizedAKI, ":", "", -1)
  
  	certificates, err := dbAccessor.GetCertificate(serial, normalizedAKI)
  	if err != nil {
  		return nil, err
  	}
  
  	if len(certificates) < 1 {
  		return nil, errors.New("no certificate found")
  	}
  
  	if len(certificates) > 1 {
  		return nil, errors.New("more than one certificate found")
  	}
  
  	return ParseCertificatePEM([]byte(certificates[0].PEM))
  }
docs: // ParseSerialNumber parses the serial number and does a lookup in the data
  // storage used for certificates. The authority key is required for the lookup
  // to work and must be passed as a hex string.