code: func FilterByAKI(aki string) (CertificateFilter, error) {
  	akiFilter, err := regexp.Compile(aki)
  	if err != nil {
  		return nil, err
  	}
  
  	return func(cm *CertificateMetadata) bool {
  		return akiFilter.MatchString(cm.AKI)
  	}, nil
  }
docs: // FilterByAKI is a CertificateFilter that returns true if the AKI in
  // a certificate matches the regular expression passed in.