code: func (u *CopyOnWriteFs) isBaseFile(name string) (bool, error) {
  	if _, err := u.layer.Stat(name); err == nil {
  		return false, nil
  	}
  	_, err := u.base.Stat(name)
  	if err != nil {
  		if oerr, ok := err.(*os.PathError); ok {
  			if oerr.Err == os.ErrNotExist || oerr.Err == syscall.ENOENT || oerr.Err == syscall.ENOTDIR {
  				return false, nil
  			}
  		}
  		if err == syscall.ENOENT {
  			return false, nil
  		}
  	}
  	return true, err
  }
docs: // Returns true if the file is not in the overlay