code: func pathListing(pfx string, children []string, out []listingItem, cb func(ch string) listingItem) []listingItem {
  	if pfx != "" {
  		pfx += "/"
  	}
  	for _, ch := range children {
  		itm := cb(ch)
  		itm.Title = strings.TrimPrefix(ch, pfx)
  		out = append(out, itm)
  	}
  	return out
  }
docs: // listing takes prefix "a/b" and children ["a/b/c", "a/b/d"] and returns
  // items titled ["c", "d"].