diff --git a/compile.rkt b/compile.rkt
index f6b8d30..8a8bf12 100644
--- a/compile.rkt
+++ b/compile.rkt
@@ -14,9 +14,10 @@
 
 ;; We try to keep at compile time a mapping from environment positions to
 ;; statically known things, to generate better code.
-(define-struct: StaticallyKnownLam ([entry-point : Symbol]
+(define-struct: StaticallyKnownLam ([name : (U Symbol False)]
+                                    [entry-point : Symbol]
                                     [arity : Natural]) #:transparent)
-(define-type CompileTimeEnvironmentEntry (U '? 'prefix StaticallyKnownLam))
+(define-type CompileTimeEnvironmentEntry (U '? Prefix StaticallyKnownLam))
 (define-type CompileTimeEnvironment (Listof CompileTimeEnvironmentEntry))
 
 
@@ -52,7 +53,7 @@
        
         (cond
           [(Top? exp)
-           (loop (Top-code exp) (cons 'prefix cenv))]
+           (loop (Top-code exp) (cons (Top-prefix exp) cenv))]
           [(Constant? exp)
            '()]
           [(LocalRef? exp)
@@ -164,7 +165,7 @@
          (append-instruction-sequences
           (make-instruction-sequence 
            `(,(make-PerformStatement (make-ExtendEnvironment/Prefix! names))))
-          (compile (Top-code top) (cons 'prefix cenv) target linkage))))
+          (compile (Top-code top) (cons (Top-prefix top) cenv) target linkage))))
 
 
 
@@ -396,19 +397,16 @@
     [(and (LocalRef? operator) (not (LocalRef-unbox? operator)))
      (let: ([static-knowledge : CompileTimeEnvironmentEntry (list-ref extended-cenv (LocalRef-depth operator))])
            (cond
-             [(eq? static-knowledge 'prefix)
-              (default)]
              [(eq? static-knowledge '?)
               (default)]
+             [(Prefix? static-knowledge)
+              (default)]
              [(StaticallyKnownLam? static-knowledge)
-              ;; Currently disabling the static analysis stuff till I get error trapping working first.
-              #;(default)
               (unless (= n (StaticallyKnownLam-arity static-knowledge))
-                (error 'arity-mismatch "Expected ~s, received ~s" 
+                (error 'arity-mismatch "~s expected ~s arguments, but received ~s" 
+                       (StaticallyKnownLam-name static-knowledge)
                        (StaticallyKnownLam-arity static-knowledge)
                        n))
-              ;; FIXME: do the arity check here...
-              #;(printf "I'm here with ~s\n" static-knowledge)
               (compile-procedure-call/statically-known-lam static-knowledge 
                                                            extended-cenv 
                                                            n
@@ -559,7 +557,8 @@
 (define (extract-static-knowledge exp cenv)
   (cond
     [(Lam? exp)
-     (make-StaticallyKnownLam (Lam-entry-label exp)
+     (make-StaticallyKnownLam (Lam-name exp)
+                              (Lam-entry-label exp)
                               (Lam-num-parameters exp))]
     [(LocalRef? exp)
      (list-ref cenv (LocalRef-depth exp))]
diff --git a/simulator-primitives.rkt b/simulator-primitives.rkt
index 7474776..67163b6 100644
--- a/simulator-primitives.rkt
+++ b/simulator-primitives.rkt
@@ -28,7 +28,7 @@
                          (syntax->list #'(name ...)))])
        (syntax/loc stx
          (let ([prim-name (make-primitive-proc 
-                           (lambda (machine return-label . args)
+                           (lambda (machine . args)
                              (apply name args)))]
                ...)
            (lambda (n)
diff --git a/simulator-structs.rkt b/simulator-structs.rkt
index 0dfb8d8..08aebc6 100644
--- a/simulator-structs.rkt
+++ b/simulator-structs.rkt
@@ -67,7 +67,7 @@
 
 
 ;; Primitive procedure wrapper
-(define-struct: primitive-proc ([f : (machine Symbol PrimitiveValue * -> PrimitiveValue)])
+(define-struct: primitive-proc ([f : (machine PrimitiveValue * -> PrimitiveValue)])
   #:transparent)
 
 
diff --git a/simulator.rkt b/simulator.rkt
index c170bac..40adcaa 100644
--- a/simulator.rkt
+++ b/simulator.rkt
@@ -309,7 +309,6 @@
                                                         (current-simulated-output-port)])
                                           (apply (primitive-proc-f prim)
                                                  m
-                                                 (ApplyPrimitiveProcedure-label op)
                                                  args))))]
                    [else
                     (error 'apply-primitive-procedure)]))]
diff --git a/test-assemble.rkt b/test-assemble.rkt
index fc97934..49cbf54 100644
--- a/test-assemble.rkt
+++ b/test-assemble.rkt
@@ -154,7 +154,7 @@
                     (make-AssignImmediateStatement (make-EnvLexicalReference 1 #f)
                                                    (make-Const 4))
                     (make-AssignPrimOpStatement 'val
-                                                (make-ApplyPrimitiveProcedure 2 'done))
+                                                (make-ApplyPrimitiveProcedure 2))
                     'done))
       "7")
 
diff --git a/test-simulator.rkt b/test-simulator.rkt
index bdcacd2..b697249 100644
--- a/test-simulator.rkt
+++ b/test-simulator.rkt
@@ -472,7 +472,7 @@
                         ,(make-PushEnvironment 2 #f)
                         ,(make-AssignImmediateStatement (make-EnvLexicalReference 0 #f) (make-Const 126389))
                         ,(make-AssignImmediateStatement (make-EnvLexicalReference 1 #f) (make-Const 42))
-                        ,(make-AssignPrimOpStatement 'val (make-ApplyPrimitiveProcedure 2 'after))
+                        ,(make-AssignPrimOpStatement 'val (make-ApplyPrimitiveProcedure 2))
                         after))])
   (test (machine-val (run m))
         (+ 126389 42))