code: func (f *BasicFilesystem) rooted(rel string) (string, error) {
  	return rooted(rel, f.root)
  }
docs: // rooted expands the relative path to the full path that is then used with os
  // package. If the relative path somehow causes the final path to escape the root
  // directory, this returns an error, to prevent accessing files that are not in the
  // shared directory.