diff --git a/bin/main.ml b/bin/main.ml
index 5c3f91b..ba34671 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -21,14 +21,12 @@ let () =
     in
     try
       let pgm = Parser.program Lexer.start lexbuf in
-      let _ = print_endline "=== Printing Input Program ===" in
+      print_endline "=== Printing Input Program ===";
       pp pgm;
-      let _ = print_endline "=== Gathering Functions ===" in
       let open Compiler in
       let env = gather_functions pgm in
-      let _ = print_endline "=== Compiling Output Program ===" in
-      let graph, _de = compile''' env Pred.Empty pgm.exp in
-      let _ = print_endline "=== Printing Output Program ===" in
+      let graph, _de = compile env Pred.Empty pgm.exp in
+      print_endline "=== Printing Output Program ===";
       Printf.printf "%s" (Graph.pp graph)
     with Parsing.Parse_error ->
       print_endline ("Parsing Error: " ^ lexbuf_contents lexbuf)
diff --git a/lib/compiler.ml b/lib/compiler.ml
index 8196065..be4c443 100644
--- a/lib/compiler.ml
+++ b/lib/compiler.ml
@@ -15,7 +15,6 @@ let gen_vertex =
 
 let rec sub (exp : Exp.t) (x : Id.t) (det_exp : Det_exp.t) : Exp.t =
   let sub' exp = sub exp x det_exp in
-  Printf.printf "factorial %s\n" x;
   match exp with
   | Int n -> Int n
   | Real r -> Real r
@@ -57,8 +56,7 @@ let gather_functions (prog : program) : Env.t =
 
 exception Not_closed_observation
 
-let compile''' (env : Env.t) (pred : Pred.t) (exp : Exp.t) : Graph.t * Det_exp.t
-    =
+let compile (env : Env.t) (pred : Pred.t) (exp : Exp.t) : Graph.t * Det_exp.t =
   let rec compile pred =
     let compile' e = compile pred e in
     let open Graph in
@@ -114,21 +112,29 @@ let compile''' (env : Env.t) (pred : Pred.t) (exp : Exp.t) : Graph.t * Det_exp.t
         let g3, det_exp_alt = compile pred_false e_alt in
         let g = g1 @+ g2 @+ g3 in
         (g, Det_exp.If (det_exp_pred, det_exp_con, det_exp_alt))
-    | Call (c, params) ->
-        let f = Env.find_exn env ~name:c in
-        let g, det_exps =
-          List.fold_map params ~init:Graph.empty ~f:(fun g e ->
-              let g', de = compile' e in
-              (g @+ g', de))
-        in
-        let { params; body; _ } = f in
-        let param_det_pairs = List.zip_exn params det_exps in
-        let sub_body =
-          List.fold param_det_pairs ~init:body
-            ~f:(fun acc (param_name, det_exp) -> sub acc param_name det_exp)
-        in
-        let g_body, det_exp_body = compile' sub_body in
-        (g @+ g_body, det_exp_body)
+    | Call (c, params) -> (
+        match Env.find env ~name:c with
+        | Some f ->
+            let g, det_exps =
+              List.fold_map params ~init:Graph.empty ~f:(fun g e ->
+                  let g', de = compile' e in
+                  (g @+ g', de))
+            in
+            let { params; body; _ } = f in
+            let param_det_pairs = List.zip_exn params det_exps in
+            let sub_body =
+              List.fold param_det_pairs ~init:body
+                ~f:(fun acc (param_name, det_exp) -> sub acc param_name det_exp)
+            in
+            let g_body, det_exp_body = compile' sub_body in
+            (g @+ g_body, det_exp_body)
+        | None ->
+            let g, det_exps =
+              List.fold_map params ~init:Graph.empty ~f:(fun g e ->
+                  let g', de = compile' e in
+                  (g @+ g', de))
+            in
+            (g, Prim_call (c, det_exps)))
     | Add (e1, e2) ->
         let g1, de1 = compile' e1 in
         let g2, de2 = compile' e2 in
diff --git a/lib/compiler.mli b/lib/compiler.mli
index d2be1bb..f25fefe 100644
--- a/lib/compiler.mli
+++ b/lib/compiler.mli
@@ -5,4 +5,4 @@ val gather_functions : Program.program -> Env.t
 
 exception Not_closed_observation
 
-val compile''' : Env.t -> Pred.t -> Program.Exp.t -> Graph.t * Program.Det_exp.t
+val compile : Env.t -> Pred.t -> Program.Exp.t -> Graph.t * Program.Det_exp.t
diff --git a/lib/env.ml b/lib/env.ml
index 672dddc..ac2e8b6 100644
--- a/lib/env.ml
+++ b/lib/env.ml
@@ -8,4 +8,4 @@ let empty : t = Map.empty (module Id)
 let add_exn (env : t) ~(name : Id.t) ~(fn : fn) =
   Map.add_exn env ~key:name ~data:fn
 
-let find_exn (env : t) ~(name : Id.t) : fn = Map.find_exn env name
+let find (env : t) ~(name : Id.t) : fn option = Map.find env name
diff --git a/lib/env.mli b/lib/env.mli
index 4abf7a4..c535c99 100644
--- a/lib/env.mli
+++ b/lib/env.mli
@@ -8,4 +8,4 @@ val add_exn :
   fn:Program.fn ->
   (string, Program.fn, Base.String.comparator_witness) Base.Map.t
 
-val find_exn : t -> name:string -> Program.fn
+val find : t -> name:string -> Program.fn option