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