let b = Button.create ~text:"Show selected composer"
    ~command:(fun () ->
      try
	let n = match (List.hd (Listbox.curselection mylist)) with
	| `Num y -> y
	| _ -> failwith "pas de sélection"
	in
	Textvariable.set v (List.nth some_composers n)
      with _ -> (print_endline "pas de sélection"; flush stdout)
	     )
    top