code: func (d *SourceFilesystem) RealDirs(from string) []string {
  	var dirnames []string
  	for _, dir := range d.Dirnames {
  		dirname := filepath.Join(dir, from)
  		if _, err := d.SourceFs.Stat(dirname); err == nil {
  			dirnames = append(dirnames, dirname)
  		}
  	}
  	return dirnames
  }
docs: // RealDirs gets a list of absolute paths to directories starting from the given
  // path.