let findi x l =
  let rec loop i n l =
    match l with
    | y::tl -> loop (i+1) (if y = x then i else n) tl
    | [] -> n
  in
  loop 0 (-1) l;;

let rindex elem =
  let rec find_index i max_found = function
    | (x::xs) when x = elem -> find_index (i+1) i xs
    | (_::xs) -> find_index (i+1) max_found xs
    | [] -> max_found
  in find_index 0 (-1);;

let rindex elem ls =
  let find_index (i, max) elem' = (i+1, if elem' = elem then i else max)
  in snd (fold_left find_index (0, -1) ls);;

let find_index elt lst =
  (* Wrap inner function that accepts an accumulator to keep the interface clean *)
  let rec find_it elt acc = function
    | hd :: tl when elt = hd -> acc (* match *)
    | hd :: tl -> find_it elt (acc + 1) tl (* non-match *)
    | _ -> raise Not_found (* end of list *)
  in find_it elt 0 lst (* call inner function with accumulator starting at 0 *)
;;