diff --git a/bin/dune b/bin/dune
index 03b8761..40c9397 100644
--- a/bin/dune
+++ b/bin/dune
@@ -1,5 +1,5 @@
 (executables
  (names main)
- (libraries incr_dom hazelnut_lib)
+ (libraries incr_dom app_lib)
  (preprocess
   (pps js_of_ocaml-ppx ppx_jane)))
diff --git a/bin/main.re b/bin/main.re
index 346a778..f0cc5bd 100644
--- a/bin/main.re
+++ b/bin/main.re
@@ -1,7 +1,7 @@
 open! Core;
 open! Incr_dom;
 open! Js_of_ocaml;
-module App = Hazelnut_lib.App;
+module App = App_lib.App;
 
 Start_app.start(
   (module App),
diff --git a/hazelnut/dune b/hazelnut/dune
new file mode 100644
index 0000000..5f2b05c
--- /dev/null
+++ b/hazelnut/dune
@@ -0,0 +1,5 @@
+(library
+ (name hazelnut_lib)
+ (libraries core incr_dom)
+ (preprocess
+  (pps ppx_jane)))
diff --git a/hazelnut/hazelnut.re b/hazelnut/hazelnut.re
new file mode 100644
index 0000000..86c5ed2
--- /dev/null
+++ b/hazelnut/hazelnut.re
@@ -0,0 +1,34 @@
+[@deriving (sexp, compare)]
+type htyp =
+  | Arrow(htyp, htyp)
+  | Num
+  | Hole;
+
+[@deriving (sexp, compare)]
+type hexp =
+  | Var(string)
+  | Lam(string, hexp)
+  | Ap(hexp, hexp)
+  | Num(int)
+  | Plus(hexp, hexp)
+  | Asc(hexp, htyp)
+  | EHole
+  | NEHole(hexp);
+
+[@deriving (sexp, compare)]
+type ztyp =
+  | Cursor(htyp)
+  | LArrow(ztyp, htyp)
+  | RArrow(htyp, ztyp);
+
+[@deriving (sexp, compare)]
+type zexp =
+  | Cursor(hexp)
+  | Lam(string, zexp)
+  | LAp(zexp, hexp)
+  | RAp(hexp, zexp)
+  | LPlus(zexp, hexp)
+  | RPlus(hexp, zexp)
+  | LAsc(zexp, htyp)
+  | RAsc(hexp, ztyp)
+  | NEHole(zexp);
diff --git a/hazelnut/hazelnut.rei b/hazelnut/hazelnut.rei
new file mode 100644
index 0000000..86c5ed2
--- /dev/null
+++ b/hazelnut/hazelnut.rei
@@ -0,0 +1,34 @@
+[@deriving (sexp, compare)]
+type htyp =
+  | Arrow(htyp, htyp)
+  | Num
+  | Hole;
+
+[@deriving (sexp, compare)]
+type hexp =
+  | Var(string)
+  | Lam(string, hexp)
+  | Ap(hexp, hexp)
+  | Num(int)
+  | Plus(hexp, hexp)
+  | Asc(hexp, htyp)
+  | EHole
+  | NEHole(hexp);
+
+[@deriving (sexp, compare)]
+type ztyp =
+  | Cursor(htyp)
+  | LArrow(ztyp, htyp)
+  | RArrow(htyp, ztyp);
+
+[@deriving (sexp, compare)]
+type zexp =
+  | Cursor(hexp)
+  | Lam(string, zexp)
+  | LAp(zexp, hexp)
+  | RAp(hexp, zexp)
+  | LPlus(zexp, hexp)
+  | RPlus(hexp, zexp)
+  | LAsc(zexp, htyp)
+  | RAsc(hexp, ztyp)
+  | NEHole(zexp);
diff --git a/lib/app.re b/lib/app.re
index fc7b750..5521e98 100644
--- a/lib/app.re
+++ b/lib/app.re
@@ -1,25 +1,34 @@
 open Core;
 open Incr_dom;
+module Hazelnut = Hazelnut_lib.Hazelnut;
+
+let render_zexp: Hazelnut.zexp => string =
+  fun
+  | Cursor(_) => "Cursor"
+  | Lam(_, _) => "Lam"
+  | LAp(_, _) => "LAp"
+  | RAp(_, _) => "RAp"
+  | LPlus(_, _) => "LPlus"
+  | RPlus(_, _) => "RPlus"
+  | LAsc(_, _) => "LAsc"
+  | RAsc(_, _) => "RAsc"
+  | NEHole(_) => "NEHole";
 
 module Model = {
   [@deriving (sexp, fields, compare)]
-  type t = {counter: int};
+  type t = {e: Hazelnut.zexp};
 
-  let set_default_input = (counter: int): t => {counter: counter};
+  let set_default_expression = (e: Hazelnut.zexp): t => {e: e};
 
-  let init = (): t => set_default_input(0);
-  let reset_counter = (_: t): t => set_default_input(0);
-  let incr_counter = (t: t): t => set_default_input(t.counter + 1);
-  let decr_counter = (t: t): t => set_default_input(t.counter - 1);
+  let init = (): t => set_default_expression(Cursor(EHole));
+  let do_nothing = (e: t): t => e;
   let cutoff = (t1: t, t2: t): bool => compare(t1, t2) == 0;
 };
 
 module Action = {
   [@deriving sexp]
   type t =
-    | Reset_counter
-    | Incr_counter
-    | Decr_counter;
+    | DoNothing;
 };
 
 module State = {
@@ -28,9 +37,7 @@ module State = {
 
 let apply_action = (model, action, _, ~schedule_action as _) =>
   switch ((action: Action.t)) {
-  | Reset_counter => Model.reset_counter(model)
-  | Incr_counter => Model.incr_counter(model)
-  | Decr_counter => Model.decr_counter(model)
+  | DoNothing => Model.do_nothing(model)
   };
 
 let on_startup = (~schedule_action as _, _) => Async_kernel.return();
@@ -52,16 +59,15 @@ let view =
     );
 
   let buttons = {
-    let reset_button = button("Reset", Action.Reset_counter);
-    let incr_button = button("+", Action.Incr_counter);
-    let decr_button = button("-", Action.Decr_counter);
+    let do_nothing_button = button("Do Nothing", Action.DoNothing);
 
-    Node.div([decr_button, reset_button, incr_button]);
+    Node.div([do_nothing_button]);
   };
 
   let%map counter = {
-    let%map counter = m >>| Model.counter;
-    Node.div([Node.textf("%d", counter)]);
+    let%map e = m >>| Model.e;
+    let e = render_zexp(e);
+    Node.div([Node.textf("%s", e)]);
   };
 
   Node.body([counter, buttons]);
diff --git a/lib/dune b/lib/dune
index 5f2b05c..d4aeefe 100644
--- a/lib/dune
+++ b/lib/dune
@@ -1,5 +1,5 @@
 (library
- (name hazelnut_lib)
- (libraries core incr_dom)
+ (name app_lib)
+ (libraries core hazelnut_lib incr_dom)
  (preprocess
   (pps ppx_jane)))
diff --git a/test/dune b/test/dune
index d7660e4..6c34aef 100644
--- a/test/dune
+++ b/test/dune
@@ -1,5 +1,5 @@
 (library
  (name hazelnut_test)
- (libraries async_kernel incr_dom_testing hazelnut_lib js_of_ocaml)
+ (libraries async_kernel incr_dom_testing app_lib js_of_ocaml)
  (preprocess
   (pps ppx_jane)))
diff --git a/test/hazelnut_test.re b/test/hazelnut_test.re
index bfae610..5ed0df9 100644
--- a/test/hazelnut_test.re
+++ b/test/hazelnut_test.re
@@ -1,6 +1,6 @@
 open! Core;
 open! Incr_dom_testing;
-module App = Hazelnut_lib.App;
+module App = App_lib.App;
 
 let make_helpers = () => {
   let driver =