code: func (ks *Keystore) TrustedKeyPrefixExists(prefix string) (bool, error) {
  	acidentifier, err := types.NewACIdentifier(prefix)
  	if err != nil {
  		return false, err
  	}
  
  	pathNamesPrefix := []string{
  		// example: /etc/rkt/trustedkeys/prefix.d/coreos.com/etcd
  		path.Join(ks.LocalPrefixPath, acidentifier.String()),
  		// example: /usr/lib/rkt/trustedkeys/prefix.d/coreos.com/etcd
  		path.Join(ks.SystemPrefixPath, acidentifier.String()),
  	}
  
  	for _, p := range pathNamesPrefix {
  		_, err := os.Stat(p)
  		if os.IsNotExist(err) {
  			continue
  		}
  		if err != nil {
  			return false, errwrap.Wrap(fmt.Errorf("cannot check dir %q", p), err)
  		}
  		files, err := ioutil.ReadDir(p)
  		if err != nil {
  			return false, errwrap.Wrap(fmt.Errorf("cannot list files in dir %q", p), err)
  		}
  		for _, f := range files {
  			if !f.IsDir() && f.Size() > 0 {
  				return true, nil
  			}
  		}
  	}
  
  	parentPrefix, _ := path.Split(prefix)
  	parentPrefix = strings.Trim(parentPrefix, "/")
  
  	if parentPrefix != "" {
  		return ks.TrustedKeyPrefixExists(parentPrefix)
  	}
  
  	return false, nil
  }
docs: // TrustKeyPrefixExists returns whether or not there exists 1 or more trusted
  // keys for a given prefix, or for any parent prefix.