code: func (b *BaseFs) RelContentDir(filename string) string {
  	for _, dirname := range b.SourceFilesystems.Content.Dirnames {
  		if strings.HasPrefix(filename, dirname) {
  			rel := strings.TrimPrefix(filename, dirname)
  			return strings.TrimPrefix(rel, filePathSeparator)
  		}
  	}
  	// Either not a content dir or already relative.
  	return filename
  }
docs: // RelContentDir tries to create a path relative to the content root from
  // the given filename. The return value is the path and language code.