code: func isInPackage(fileName string, pkg string) (bool, error) {
  	dir, err := filepath.Abs(filepath.Dir(fileName))
  	if err != nil {
  		return false, err
  	}
  	dir = path.Clean(dir)
  	pkg = path.Clean(pkg)
  	if !strings.HasSuffix(dir, pkg) {
  		return false, nil
  	}
  
  	src := strings.TrimSuffix(dir, pkg)
  	src = path.Clean(src)
  	goPaths := strings.Split(os.Getenv("GOPATH"), string(filepath.ListSeparator))
  	for _, goPath := range goPaths {
  		if filepath.Join(goPath, "src") == src {
  			return true, nil
  		}
  	}
  	return false, nil
  }
docs: // isInPackage returns true if the filename is a part of the package.