diff --git a/dune b/dune
index e18232f64..88809917d 100644
--- a/dune
+++ b/dune
@@ -1,4 +1,4 @@
 
-(executable
-  (name test)
+(executables
+  (names test fib)
   (libraries lwt lwt.unix))
diff --git a/fib.ml b/fib.ml
new file mode 100644
index 000000000..6e43941b7
--- /dev/null
+++ b/fib.ml
@@ -0,0 +1,27 @@
+
+open Lwt.Syntax
+
+let suspend f =
+  let* () = Lwt.pause() in
+  f()
+
+let rec sleepy_fib n =
+  if n <= 2 then (
+    Lwt.await @@ Lwt_unix.sleep 0.2;
+    Lwt.return 1
+  ) else (
+    let f1 = sleepy_fib (n-1) in
+    let f2 = sleepy_fib (n-2) in
+    Lwt.await @@ Lwt_unix.sleep 0.2;
+    let res = Lwt.await f1 + Lwt.await f2 in
+    Printf.printf "fib %d = %d\n%!" n res;
+    Lwt.return res
+  )
+
+let main () =
+  suspend @@ fun () ->
+  let _res = Lwt.await @@ sleepy_fib 10 in
+  Lwt.return()
+
+let () =
+  Lwt_main.run @@ main ()