diff --git a/assemble.rkt b/assemble.rkt
index cf30f60..7767f50 100644
--- a/assemble.rkt
+++ b/assemble.rkt
@@ -260,7 +260,9 @@ EOF
     [(eq? target 'val)
      "MACHINE.val"]
     [(EnvLexicalReference? target)
-     (assemble-lexical-reference target)]))
+     (assemble-lexical-reference target)]
+    [(EnvPrefixReference? target)
+     (assemble-prefix-reference target)]))
 
 
 
@@ -288,11 +290,18 @@ EOF
   (format "MACHINE.env[MACHINE.env.length - 1 - ~a]"
           (EnvLexicalReference-depth a-lex-ref)))
 
+(: assemble-prefix-reference (EnvPrefixReference -> String))
+(define (assemble-prefix-reference a-ref)
+  (format "MACHINE.env[MACHINE.env.length - 1 - ~a][~a]"
+          (EnvPrefixReference-depth a-ref)
+          (EnvPrefixReference-pos a-ref)))
+
 (: assemble-whole-prefix-reference (EnvWholePrefixReference -> String))
 (define (assemble-whole-prefix-reference a-prefix-ref)
   (format "MACHINE.env[MACHINE.env.length - 1 - ~a]"
           (EnvWholePrefixReference-depth a-prefix-ref)))
 
+
 (: assemble-env-reference (EnvReference -> String))
 (define (assemble-env-reference ref)
   (cond
diff --git a/compile.rkt b/compile.rkt
index 47fd175..52e9a0b 100644
--- a/compile.rkt
+++ b/compile.rkt
@@ -418,7 +418,10 @@
                     'proc]
                    [(EnvLexicalReference? target)
                     ;; The optimization is right here.
-                    (make-EnvLexicalReference (+ (EnvLexicalReference-depth target) n))])
+                    (make-EnvLexicalReference (+ (EnvLexicalReference-depth target) n))]
+                   [(EnvPrefixReference? target)
+                    ;; The optimization is right here.
+                    (make-EnvPrefixReference (+ (EnvPrefixReference-depth target) n) (EnvPrefixReference-pos target))])
              (make-ApplyPrimitiveProcedure n after-call))
            ,(make-PopEnvironment n 0))))
 
diff --git a/il-structs.rkt b/il-structs.rkt
index 4cb246c..bd618d1 100644
--- a/il-structs.rkt
+++ b/il-structs.rkt
@@ -26,7 +26,9 @@
 
 
 ;; Targets: these are the allowable lhs's for an assignment.
-(define-type Target (U AtomicRegisterSymbol EnvLexicalReference))
+(define-type Target (U AtomicRegisterSymbol 
+                       EnvLexicalReference
+                       EnvPrefixReference))
 
 
 
@@ -38,10 +40,14 @@
   #:transparent)
 (define-struct: EnvLexicalReference ([depth : Natural])
   #:transparent)
+(define-struct: EnvPrefixReference ([depth : Natural]
+                                    [pos : Natural])
+  #:transparent)
 (define-struct: EnvWholePrefixReference ([depth : Natural])
   #:transparent)
 
 
+
 ;; An environment reference is either lexical or referring to a whole prefix.
 (define-type EnvReference (U EnvLexicalReference
                              EnvWholePrefixReference))
diff --git a/simulator.rkt b/simulator.rkt
index 182abbc..3082b73 100644
--- a/simulator.rkt
+++ b/simulator.rkt
@@ -91,7 +91,12 @@
               [(eq? t 'val)
                (val-update m v)]
               [(EnvLexicalReference? t)
-               (env-mutate m (EnvLexicalReference-depth t) v)])))
+               (env-mutate m (EnvLexicalReference-depth t) v)]
+              [(EnvPrefixReference? t)
+               (toplevel-mutate! (ensure-toplevel (env-ref m (EnvPrefixReference-depth t)))
+                                 (EnvPrefixReference-pos t)
+                                 (ensure-primitive-value v))
+               m])))
 
 
 (: step-push-environment (machine PushEnvironment -> machine))
@@ -206,7 +211,13 @@
      val-update]
     [(EnvLexicalReference? t)
      (lambda: ([m : machine] [v : SlotValue])
-              (env-mutate m (EnvLexicalReference-depth t) v))]))
+              (env-mutate m (EnvLexicalReference-depth t) v))]
+    [(EnvPrefixReference? t)
+     (lambda: ([m : machine] [v : SlotValue])
+              (toplevel-mutate! (ensure-toplevel (env-ref m (EnvPrefixReference-depth t)))
+                                (EnvPrefixReference-pos t)
+                                (ensure-primitive-value v))
+              m)]))
 
 
 (: step-assign-primitive-operation (machine AssignPrimOpStatement -> machine))