code: func ProtosBelow(dirs []string) ([]string, error) {
  	protos := []string{}
  	for _, dir := range dirs {
  		err := filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {
  			if err != nil {
  				return err
  			}
  			if !info.IsDir() && strings.HasSuffix(info.Name(), ".proto") {
  				protos = append(protos, path)
  			}
  			return nil
  		})
  		if err != nil {
  			return nil, err
  		}
  	}
  	return protos, nil
  }
docs: // ProtosBelow returns a slice containing the filenames of all .proto
  // files found in or below the given directories.