code: func (s Store) TmpNamedFile(name string) (*os.File, error) {
  	dir, err := s.TmpDir()
  	if err != nil {
  		return nil, err
  	}
  	fname := filepath.Join(dir, name)
  	_, err = os.Stat(fname)
  	if os.IsNotExist(err) {
  		return os.Create(fname)
  	}
  	if err != nil {
  		return nil, err
  	}
  	return os.OpenFile(fname, os.O_RDWR|os.O_APPEND, 0644)
  }
docs: // TODO(sgotti), unexport this and provide other functions for external users
  // TmpNamedFile returns an *os.File with the specified name local to the same
  // filesystem as the Store, or any error encountered. If the file already
  // exists it will return the existing file in read/write mode with the cursor
  // at the end of the file.