diff --git a/sexpr.ml b/sexpr.ml
index a0ba3da..f439f42 100644
--- a/sexpr.ml
+++ b/sexpr.ml
@@ -1,15 +1,19 @@
-module Testing = struct
-  type result = Pass | Fail | Skip
+module type Testing = sig
+  val test : string -> ?depends:(string list) -> (unit -> unit) -> unit;;
+end;;
 
-  let testing = ref false
-  let test_only = ref []
-  let test_results = ref []
+module Testing : Testing = struct
+  type result = Pass | Fail | Skip;;
+
+  let testing = ref false;;
+  let test_only = ref [];;
+  let test_results = ref [];;
 
   let rec should_skip test =
     match List.assoc_opt test !test_results with
       Some Fail
     | Some Skip -> true
-    | _         -> false
+    | _         -> false;;
 
   let test name ?(depends = []) run =
     let test_only = !test_only in
@@ -33,7 +37,7 @@ module Testing = struct
           Fail
       end; in
       test_results := (name, res) :: !test_results
-    end
+    end;;
 
   let () =
     let rec proc args =
@@ -49,9 +53,9 @@ module Testing = struct
       | hd :: tl ->
           proc tl
       end
-    in proc @@ Array.to_list Sys.argv
-end
-let test = Testing.test
+    in proc @@ Array.to_list Sys.argv;;
+end;;
+let test = Testing.test;;
 
 
 module PC = struct