(* sum : int list -> int *)
let rec sum is = match is with
  | [] -> 0
  | i::is -> i + sum is

(* sum' : int list -> int ---- the tail-call/accumulator version *)
let sum' is =
  let rec visit is acc = match is with
    | [] -> acc
    | i::is ->
      visit is (i + acc) in
  visit is 0

(* append : 'a list -> 'a list -> 'a list *)
let rec append xs ys = match xs with
  | [] -> ys
  | x::xs -> x :: (append xs ys)

type inst =
  | Push of int
  | Add
  | Mult

(* instructions_to_string : inst list -> string *)
let rec instructions_to_string is =
  let i2s i = match i with
    | Push n -> "push " ^ (string_of_int n) ^ "; "
    | Add    -> "add; "
    | Mult   -> "mult; "
  in
  match is with
    | [] -> ""
    | i::is->
      let svi  = i2s i in
      let svis = instructions_to_string is in
      svi ^ svis