(* 10th function *)
fun traverse f_c f_v f_acc acc exp =
    case exp of
        Constant(x) => f_acc (f_c x, acc)
        | Variable(x) => f_acc (f_v x, acc)
        | Pair([x1,x2]) => f_acc (traverse f_c f_v f_acc acc x1, traverse f_c f_v f_acc acc x2)
        | Operator(_,exp1) => traverse f_c f_v f_acc acc exp1
        | List(head::tail) => f_acc (traverse f_c f_v f_acc acc head, traverse f_c f_v f_acc acc (List(tail)))
        | _ => acc

(* 7th function *)
fun count_constants exp = traverse (fn x=>1) (fn x=>0) op+ 0 exp

(* 8th function *)
fun sum_constants exp = traverse (fn x=>x) (fn x=>0) op+ 0 exp

(* 9th function *)
fun all_variables exp = traverse (fn x=>[]) (fn x=>[x]) (fn (x,acc) => (List.filter (fn y=>null x orelse y<>hd x) acc) @ x) [] exp