code: func (sc ScatterGather) Add(wd string, rel string) error {
  	cleaned := filepath.Clean(rel)
  	if _, ok := sc[cleaned]; ok {
  		return errors.Reason("name conflict %q", rel).Err()
  	}
  	sc[cleaned] = wd
  	return nil
  }
docs: // Add adds a (working directory, relative path) pair to the ScatterGather.
  //
  // Add returns an error if the relative path was already added.