test.ml

let filter xs =
  List.filter (function `G a -> true | _ -> false) xs

let rec refine (xs : [> `G of 'a] list) : [`G of 'a] list =
  match xs with
  | [] -> []
  | ((`G _) as elt)::rest -> elt::refine rest
  | _::_ -> assert false

let argtest = [`X(`True);`G(`False)];;

let toto = refine (filter argtest)