code: func (ff FilesystemView) RelativePath(path string) (string, error) {
  	relPath, err := filepath.Rel(ff.root, path)
  	if err != nil {
  		return "", fmt.Errorf("calculating relative path(%q): %v", path, err)
  	}
  	if ff.skipRelPath(relPath) {
  		relPath = ""
  	} else if ff.sourcePrefix != "" {
  		relPath = filepath.Join(ff.sourcePrefix, relPath)
  	}
  	return relPath, nil
  }
docs: // RelativePath returns a version of path which is relative to the FilesystemView root,
  // or an empty string if path matches a blacklist entry.
  //
  // Blacklist globs are matched against the entirety of each of:
  //  * the path relative to the FilesystemView root.
  //  * the basename return by filepath.Base(path).
  // See filepath.Match for details about the format of blacklist globs.