diff --git a/impls/sml/env.sml b/impls/sml/env.sml
index 5e4adcd0..91814e15 100644
--- a/impls/sml/env.sml
+++ b/impls/sml/env.sml
@@ -1,5 +1,13 @@
-datatype mal_env = ENV of (string * mal_type) list
+type mal_defs = (string * mal_type) list
 
-fun lookup (ENV fs) s =
-    fs |> List.find (eq s o #1)
-       |> Option.map #2
+datatype mal_env = ENV of mal_defs
+                 | INNER of (mal_defs * mal_env)
+
+fun get (d:mal_defs) s =
+    d |> List.find (eq s o #1) |> Option.map #2
+
+fun set s v (d:mal_defs) =
+    (s, v) :: (d |> List.filter (not o eq s o #1))
+
+fun lookup (INNER (d, out)) s = (case get d s of NONE => lookup out s | x => x)
+  | lookup (ENV d)          s = get d s
diff --git a/impls/sml/step3_env.sml b/impls/sml/step3_env.sml
index 93972570..e9b562f2 100644
--- a/impls/sml/step3_env.sml
+++ b/impls/sml/step3_env.sml
@@ -40,20 +40,20 @@ fun malMinus (INT b, INT a) = INT (a - b)
 fun malDiv   (INT b, INT a) = INT (a div b)
   | malDiv _ = raise NotApplicable "can only divide integers"
 
-val initEnv = ENV [
-    ("+", FN (foldl malPlus (INT 0))),
-    ("*", FN (foldl malTimes (INT 1))),
-    ("-", FN (
-        fn [x]   => malMinus (x, INT 0)
-         | x::xs => foldr malMinus x xs
-         | _ => raise NotApplicable "'-' requires at least one argument"
-    )),
-    ("/", FN (
-        fn [x]   => malDiv (x, INT 1)
-         | x::xs => foldr malDiv x xs
-         | _ => raise NotApplicable "'/' requires at least one argument"
-    ))
-]
+val initEnv =
+  [] |> set "+"
+        (FN (foldl malPlus (INT 0)))
+     |> set "*"
+        (FN (foldl malTimes (INT 1)))
+     |> set "-"
+        (FN (fn [x]   => malMinus (x, INT 0)
+              | x::xs => foldr malMinus x xs
+              | _     => raise NotApplicable "'-' requires arguments"))
+     |> set "/"
+        (FN (fn [x]   => malDiv (x, INT 1)
+              | x::xs => foldr malDiv x xs
+              | _     => raise NotApplicable "'/' requires arguments"))
+     |> ENV
 
 fun repl () =
     let open TextIO