let element_of_xml ?expected xml =
  let open Webdav in
  try
    let n = Xml.tag xml in
    begin
      match expected with
      | None ->
        Ok n
      | Some expected ->
        if String.compare n expected = 0 then begin
          Ok n
        end else begin
          Error (`Bad_request, sprintf "Expected <%s>, got <%s>" expected n)
        end
    end
  with Xml.Not_element _ ->
    Error (`Bad_request,
           sprintf "Expected an element, got %s" (Xml.to_string xml))

let webdav_result_map f = function
  | Webdav.Error _ as err -> err
  | Webdav.Ok x -> f x

let propfind_request_of_prop prop =
  let open Webdav in
  List.fold_left (fun acc ch ->
      webdav_result_map (fun acc ->
          webdav_result_map (fun x ->
              Ok (x :: acc) ) (element_of_xml ch)) acc) (Ok []) (Xml.children prop)