diff --git a/aoc2020.ml b/aoc2020.ml
index 5d6d4ab..b2efebe 100644
--- a/aoc2020.ml
+++ b/aoc2020.ml
@@ -5,4 +5,4 @@ let days = [|Day1.main; Day2.main; Day3.main|]
 let _ =
   match Array.to_list Sys.argv with
   | _exename :: day :: args -> days.(int_of_string day - 1) args
-  | _ -> Usage.exit "<day> args..."
+  | _ -> Misc.usage "<day> args..."
diff --git a/day1.ml b/day1.ml
index 8350af9..a2b8b4b 100644
--- a/day1.ml
+++ b/day1.ml
@@ -1,13 +1,4 @@
-module IntSet = struct
-  include Set.Make(Int)
-
-  let exists_opt (type a) (f: int -> a option) (set: t): a option =
-    let exception Found of a in
-    let f' x = Option.iter (fun x -> raise (Found x)) (f x) in
-    match iter f' set with
-    | exception Found x -> Some x
-    | _                 -> None
-end
+module IntSet = Set_ext.Make(Int)
 
 let read_ints =
   Bracket.infile_lines ~line:int_of_string ~of_seq:IntSet.of_seq
@@ -23,31 +14,18 @@ let rec find ?(target=2020) ~n set =
       Option.map (List.cons x) (find ~target:(target - x) ~n:(n - 1) set) in
     IntSet.exists_opt joined set
 
-let print_result xs =
-  let res = List.fold_left ( * ) 1 xs in
-  let rec print_prod fmt = function
-    | []    -> Format.pp_print_text fmt "{}"
-    | [x]   -> Format.fprintf fmt "%d" x
-    | x::xs -> Format.fprintf fmt "%d * %a" x print_prod xs in
-  Format.printf "%a = %d\n" print_prod xs res
-
-
 let main' n input =
   match find ~n (read_ints input) with
-  | Some xs -> print_result xs
+  | Some xs -> Misc.print_prod xs
   | None    -> raise Not_found
 
-let mains = [|main' 2; main' 3|]
-
-let main = function
-  | [part; input] -> mains.(int_of_string part - 1) input
-  | _             -> Usage.exit_default 1
+let main = Misc.main 1 [|main' 2; main' 3|]
 
 
 let%expect_test "part 1" =
-  mains.(0) "../../data/day1"; (* is this always the right path??? *)
+  main' 2 "../../data/day1"; (* is this always the right path??? *)
   [%expect{| 534 * 1486 = 793524 |}]
 
 let%expect_test "part 2" =
-  mains.(1) "../../data/day1";
+  main' 3 "../../data/day1";
   [%expect{| 71 * 686 * 1263 = 61515678 |}]
diff --git a/day2.ml b/day2.ml
index 4c3674c..dfba071 100644
--- a/day2.ml
+++ b/day2.ml
@@ -26,17 +26,13 @@ let main' ok input =
   Bracket.infile_iter_lines input ~line:parse_line ~iter:check;
   Printf.printf "%d out of %d ok\n" !count_ok !count_all
 
-let mains = [|main' ok1; main' ok2|]
-
-let main = function
-  | [part; input] -> mains.(int_of_string part - 1) input
-  | _             -> Usage.exit_default 2
+let main = Misc.main 2 [|main' ok1; main' ok2|]
 
 
 let%expect_test "part 1" =
-  mains.(0) "../../data/day2";
+  main' ok1 "../../data/day2";
   [%expect{| 607 out of 1000 ok |}]
 
 let%expect_test "part 2" =
-  mains.(1) "../../data/day2";
+  main' ok2 "../../data/day2";
   [%expect{| 321 out of 1000 ok |}]
diff --git a/day3.ml b/day3.ml
index 5530f03..e326a9e 100644
--- a/day3.ml
+++ b/day3.ml
@@ -36,19 +36,14 @@ let main1 input =
 
 let main2 input =
   let map = read_file input in
-  let prod = List.fold_left ( * ) 1
+  Misc.print_prod
     [count_trees map;
      count_trees map ~step_y:3;
      count_trees map ~step_y:5;
      count_trees map ~step_y:7;
-     count_trees map ~step_x:2] in
-  Format.printf "product: %d\n" prod
+     count_trees map ~step_x:2]
 
-let mains = [|main1; main2|]
-
-let main = function
-  | [part; input] -> mains.(int_of_string part - 1) input
-  | _             -> Usage.exit_default 3
+let main = Misc.main 3 [|main1; main2|]
 
 let%expect_test _ =
   main1 "../../data/day3";
@@ -56,4 +51,4 @@ let%expect_test _ =
 
 let%expect_test _ =
   main2 "../../data/day3";
-  [%expect{| product: 9406609920 |}]
+  [%expect{| 90 * 244 * 97 * 92 * 48 = 9406609920 |}]
diff --git a/dune b/dune
index 868d48e..5346368 100644
--- a/dune
+++ b/dune
@@ -7,4 +7,4 @@
   (public_name aoc2020)
   (inline_tests)
   (modules (:standard \ aoc2020))
-  (preprocess (pps ppx_expect)))
+  (preprocess (pps ppx_deriving.std ppx_expect)))
diff --git a/misc.ml b/misc.ml
new file mode 100644
index 0000000..206f5f6
--- /dev/null
+++ b/misc.ml
@@ -0,0 +1,31 @@
+let usage args =
+  let err = Format.sprintf "usage: %s %s" Sys.argv.(0) args in
+  print_endline err;
+  exit 1
+
+let usage_default day = usage (string_of_int day ^ " <part> <infile>")
+
+let main day mains = function
+  | [part; input] -> mains.(int_of_string part - 1) input
+  | _             -> usage_default day
+
+
+type 'a monoid =
+  {id: 'a;
+   op: 'a -> 'a -> 'a;
+   op_name: string;
+   pp: Format.formatter -> 'a -> unit}
+
+let mult = {id = 1; op = ( * ); op_name = "*"; pp = Format.pp_print_int}
+let add  = {id = 0; op = ( + ); op_name = "+"; pp = Format.pp_print_int}
+
+let print_fold mon xs =
+  let res = List.fold_left mon.op mon.id xs in
+  let rec go fmt = function
+    | []    -> Format.fprintf fmt "%a" mon.pp mon.id
+    | [x]   -> Format.fprintf fmt "%a" mon.pp x
+    | x::xs -> Format.fprintf fmt "%a * %a" mon.pp x go xs in
+  Format.printf "%a = %a\n" go xs mon.pp res
+
+let print_prod = print_fold mult
+let print_sum  = print_fold add
diff --git a/misc.mli b/misc.mli
new file mode 100644
index 0000000..f13fa86
--- /dev/null
+++ b/misc.mli
@@ -0,0 +1,16 @@
+val usage: string -> 'a
+val usage_default: int -> 'a
+
+val main: int -> (string -> unit) array -> string list -> unit
+
+type 'a monoid =
+  {id: 'a;
+   op: 'a -> 'a -> 'a;
+   op_name: string;
+   pp: Format.formatter -> 'a -> unit}
+val mult: int monoid
+val add: int monoid
+
+val print_fold: 'a monoid -> 'a list -> unit
+val print_prod: int list -> unit
+val print_sum:  int list -> unit
diff --git a/set_ext.ml b/set_ext.ml
new file mode 100644
index 0000000..2920ad0
--- /dev/null
+++ b/set_ext.ml
@@ -0,0 +1,15 @@
+module type S = sig
+  include Set.S
+  val exists_opt: (elt -> 'a option) -> t -> 'a option
+end
+
+module Make(Elt: Set.OrderedType) = struct
+  include Set.Make(Elt)
+
+  let exists_opt (type a) (f: elt -> a option) set =
+    let exception Found of a in
+    let f' x = Option.iter (fun x -> raise (Found x)) (f x) in
+    match iter f' set with
+    | exception Found x -> Some x
+    | _                 -> None
+end
diff --git a/set_ext.mli b/set_ext.mli
new file mode 100644
index 0000000..7702815
--- /dev/null
+++ b/set_ext.mli
@@ -0,0 +1,8 @@
+module type S = sig
+  include Set.S
+  val exists_opt: (elt -> 'a option) -> t -> 'a option
+end
+
+module Make(Elt: Set.OrderedType): S
+  with type elt = Elt.t
+   and type t   = Set.Make(Elt).t
diff --git a/usage.ml b/usage.ml
deleted file mode 100644
index f572b7b..0000000
--- a/usage.ml
+++ /dev/null
@@ -1,6 +0,0 @@
-let exit args =
-  let err = Format.sprintf "usage: %s %s" Sys.argv.(0) args in
-  print_endline err;
-  exit 1
-
-let exit_default day = exit (string_of_int day ^ " <part> <infile>")