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