type FileSys = Element list
and Element =
| File of string*string
| Dir of string * FileSys;;


let d1 = Dir("d1",[File("a1","java");
Dir("d2", [File("a2","fsx");
Dir("d3", [File("a3","fs")])]);
File("a4","fsx");
Dir("d3", [File("a5","pdf")])]);;

// 1)
let rec namesFileSys = function
    | [] -> []
    | e::es -> (namesElement e) @ (namesFileSys es)
    and namesElement = function
    | File(s,ss) -> [s+"."+ss]
    | Dir(s,fs) -> s :: (namesFileSys fs);;

// 2)
let rec searchFileSys = function
| [] -> []
| e::es -> (namesElement e) @ (namesFileSys es)
and searchElement = function
| File(s,ss) ->
| Dir(s,fs) -> s :: (searchFileSys fs)