let rec odd_elements l =
  match l with
  | a::_::t -> a :: odd_elements t
  | _ -> l;;

let int_odd_elements : int list -> int list = even_elements;;

let rec even_elements l =
  match l with
  | _::a::t -> a :: even_elements t
  | _ -> [];;

let int_even_elements : int list -> int list = even_elements;;

(*
#trace int_even_elements;;
int_even_elements [2; 4; 2; 4; 2];;
int_even_elements <-- [2; 4; 2; 4; 2]
int_even_elements <-- [2; 4; 2]
int_even_elements <-- [2]
int_even_elements --> []
int_even_elements --> [4]
int_even_elements --> [4; 4]
- : int list = [4; 4]
*)