code: func (bk *Backend) flatten(bucket []string) string {
  	// Convert ["foo", "bar"] to "foo/bar"
  	raw := filepath.Join(bucket...)
  
  	// URL encode bucket from "foo/bar" to "foo%2Fbar".
  	flat := url.QueryEscape(raw)
  
  	return filepath.Join(bk.rootDir, flat)
  }
docs: // flatten takes a bucket and flattens it (URL encodes) and prepends the root
  // directory returning the full path to the bucket on the filesystem.