diff --git a/simulator.rkt b/simulator.rkt
index 7058e2e..6e9ef19 100644
--- a/simulator.rkt
+++ b/simulator.rkt
@@ -139,7 +139,12 @@
   (let: ([op : PrimitiveCommand (PerformStatement-op stmt)])
         (cond
           [(SetToplevel!? op)
-           (error 'step-perform)]
+           (env-mutate m 
+                       (SetToplevel!-depth op)
+                       (toplevel-mutate (ensure-toplevel (env-ref m (SetToplevel!-depth op)))
+                                        (SetToplevel!-pos op)
+                                        
+                                        (machine-val m)))]
           [(CheckToplevelBound!? op)
            (error 'step-perform)]
           [(ExtendEnvironment/Prefix!? op)
@@ -182,7 +187,15 @@
     [else
      (error 'ensure-symbol)]))
   
-  
+
+(: ensure-toplevel (Any -> toplevel))
+(define (ensure-toplevel v)
+  (cond
+    [(toplevel? v)
+     v]
+    [else
+     (error 'ensure-toplevel)]))
+
 
 (: current-instruction (machine -> Statement))
 (define (current-instruction m)
@@ -280,3 +293,9 @@
            (loop (add1 i))])))
 
 
+(: toplevel-mutate (toplevel Natural Any -> toplevel))
+(define (toplevel-mutate a-top index v)
+  (make-toplevel (append (take (toplevel-vals a-top) index)
+                         (list v)
+                         (drop (toplevel-vals a-top) (add1 index)))))
+                 
\ No newline at end of file
diff --git a/test-simulator.rkt b/test-simulator.rkt
index 8f1bbef..c3a121f 100644
--- a/test-simulator.rkt
+++ b/test-simulator.rkt
@@ -230,16 +230,25 @@
                                  (lookup-primitive '-)
                                  (lookup-primitive '*)
                                  (lookup-primitive '=)))))
-
-
 (let ([m (new-machine `(,(make-PerformStatement (make-ExtendEnvironment/Prefix! '(some-variable)))
                         ,(make-AssignImmediateStatement 'val (make-Const "Danny"))
                         ,(make-PerformStatement (make-SetToplevel! 0 0 'some-variable))))])
   (run m)
   ;; FIXME:  I'm hitting what appears to be a Typed Racket bug that prevents me from inspecting
   ;; the toplevel structure in the environment... :(
-  #;(test (first (machine-env (run m)))
-          (make-toplevel (vector (lookup-primitive '+)
-                                 (lookup-primitive '-)
-                                 (lookup-primitive '*)
-                                 (lookup-primitive '=)))))
\ No newline at end of file
+  )
+(let ([m (new-machine `(,(make-PerformStatement (make-ExtendEnvironment/Prefix! '(some-variable another)))
+                        ,(make-AssignImmediateStatement 'val (make-Const "Danny"))
+                        ,(make-PerformStatement (make-SetToplevel! 0 1 'another))))])
+  (run m)
+  ;; FIXME:  I'm hitting what appears to be a Typed Racket bug that prevents me from inspecting
+  ;; the toplevel structure in the environment... :(
+  )
+(let ([m (new-machine `(,(make-PerformStatement (make-ExtendEnvironment/Prefix! '(some-variable)))
+                        ,(make-AssignImmediateStatement 'val (make-Const "Danny"))
+                        ,(make-PushEnvironment 5)
+                        ,(make-PerformStatement (make-SetToplevel! 5 0 'some-variable))))])
+  (run m)
+  ;; FIXME:  I'm hitting what appears to be a Typed Racket bug that prevents me from inspecting
+  ;; the toplevel structure in the environment... :(
+  )