set beresp.http.Surrogate-Key = ""
  if(beresp.http.Surrogate-Key, beresp.http.Surrogate-Key, "") // Retain existing keys set by origin server
  if(beresp.http.cache-control ~ "immutable", "", " all") // Add the "all" key if the response is not immutable
  " " req.url.path // Add the URL path as a key
;