JJ: Description from the destination commit:

add error for unsupported CS denom

JJ: Description from source commit:
better fix

.jjconflict-side-0/src/headers.ml

@@ -0,0 +1,52 @@
+let pp_array pp_item item = Fmt.array ~sep:(Fmt.any ", ") pp_item item
+
+let accept_header_value =
+  let s =
+    Fmt.str "%a"
+      (pp_array Assets.Mimetype.pp_mime)
+      Assets.supported_mimetype_arr
+  in
+  s
+
+let avail_languages_header_value =
+  let s = Fmt.str "%a" (pp_array Fmt.string) Assets.supported_lang_arr in
+  s
+
+let select_mimetype headers =
+  let accept = Vif.Headers.get headers "accept" in
+  Cohttp.Accept.media_ranges accept
+  |> Cohttp.Accept.qsort
+  |> List.filter_map (fun (_q, (m, _p)) -> Assets.Mimetype.of_cohttp_media m)
+  |> List.find_opt Assets.is_supported_mimetype
+
+let select_language headers =
+  let accept_language = Vif.Headers.get headers "accept-language" in
+  Cohttp.Accept.languages accept_language
+  |> Cohttp.Accept.qsort
+  |> List.map (fun (_q, lang) -> lang)
+  |> List.map (function
+    | Cohttp.Accept.AnyLanguage -> Assets.default_lang
+    | Language language_range -> (
+        (* ignore language subtags (e.g. "en-US" -> "en") *)
+        match language_range with
+        | [] -> assert false
+        | primary_tag :: _ -> primary_tag))
+  |> List.find_opt Assets.is_supported_lang
+  |> Option.value ~default:Assets.default_lang
+
+let select_encoding headers =
+  Vif.Headers.get headers "accept-encoding"
+  |> Cohttp.Accept.encodings
+  |> Cohttp.Accept.qsort
+  |> List.map snd
+  |> List.filter_map (function
+    | Cohttp.Accept.Identity -> Some `Identity
+    | Deflate -> Some `DEFLATE
+    | Gzip -> Some `Gzip
+    | AnyEncoding -> Some Assets.default_encoding
+    | Encoding _ | Compress -> (* unsupported *) None)
+  |> function
+  | [] -> assert false
+  | `Identity :: _ -> None
+  | `DEFLATE :: _ -> Some `DEFLATE
+  | `Gzip :: _ -> Some `Gzip