code: func URLPart(ctx context.Context, r *http.Request, i int) (context.Context, string, bool) {
  	ctx, parts := URLParts(ctx, r)
  	if len(parts) > i && i >= 0 {
  		return ctx, parts[i], true
  	}
  	return ctx, "", false
  }
docs: // URLPart returns a part of the url and caches it in the context