diff --git a/dune b/dune
index 5346368..ef2095f 100644
--- a/dune
+++ b/dune
@@ -7,4 +7,4 @@
   (public_name aoc2020)
   (inline_tests)
   (modules (:standard \ aoc2020))
-  (preprocess (pps ppx_deriving.std ppx_expect)))
+  (preprocess (pps ppx_deriving.std ppx_inline_test ppx_expect)))
diff --git a/misc.ml b/misc.ml
index 206f5f6..093a1ef 100644
--- a/misc.ml
+++ b/misc.ml
@@ -19,13 +19,44 @@ type 'a monoid =
 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 fold_list {op; id; _} = List.fold_left op id
+
+let%test _ = fold_list add  [] = 0
+let%test _ = fold_list add  [1;2;3;4] = 10
+let%test _ = fold_list mult [1;2;3;4] = 24
+
+
 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
+    | x::xs -> Format.fprintf fmt "%a %s %a" mon.pp x mon.op_name 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
+
+let%expect_test _ =
+  print_prod [];
+  [%expect{| 1 = 1 |}]
+
+let%expect_test _ =
+  print_prod [1];
+  [%expect{| 1 = 1 |}]
+
+let%expect_test _ =
+  print_prod [1;2;3;4];
+  [%expect{| 1 * 2 * 3 * 4 = 24 |}]
+
+let%expect_test _ =
+  print_sum [];
+  [%expect{| 0 = 0 |}]
+
+let%expect_test _ =
+  print_sum [1];
+  [%expect{| 1 = 1 |}]
+
+let%expect_test _ =
+  print_sum [1;2;3;4];
+  [%expect{| 1 + 2 + 3 + 4 = 10 |}]
diff --git a/misc.mli b/misc.mli
index f13fa86..cb59edb 100644
--- a/misc.mli
+++ b/misc.mli
@@ -8,6 +8,9 @@ type 'a monoid =
    op: 'a -> 'a -> 'a;
    op_name: string;
    pp: Format.formatter -> 'a -> unit}
+
+val fold_list: 'a monoid -> 'a list -> 'a
+
 val mult: int monoid
 val add: int monoid