Longnames

let rec longNamesFileSys = function
    | [] -> Set.empty
    | e :: es -> Set.union (longNamesElement e) (longNamesFileSys es)
and longNamesElement = function
    | File(s,ext) -> Set.ofList [s + "." + ext]
    | Dir(s,fs) ->  Set.map (fun x -> s + "/" + x) (longNamesFileSys fs)