diff --git a/collects/htdp/error.ss b/collects/htdp/error.ss
index d6f49cabdf..8ec79454e8 100644
--- a/collects/htdp/error.ss
+++ b/collects/htdp/error.ss
@@ -1,74 +1,81 @@
-#cs(module error mzscheme
-  (require (lib "etc.ss") (lib "list.ss"))
-  ;; --------------------------------------------------------------------------
-  (provide check-arg check-arity check-proc check-result check-list-list find-non
-           tp-exn? number->ord)
+#lang mzscheme
 
-  ;; (_ -> Boolean) (listof X) -> (union X false)
-  (define (find-non pred? l)
-    (let ([r (filter (compose not pred?) l)])
-      (if (null? r) #f (car r))))
+(require (lib "etc.ss") 
+         (lib "list.ss"))
 
-  #| Tests ------------------------------------------------------------------
+;; --------------------------------------------------------------------------
+(provide check-arg check-arity check-proc check-result check-list-list
+         natural?
+         find-non tp-exn? number->ord)
+
+(define (natural? w)
+  (and (number? w) (integer? w) (>= w 0)))
+
+;; (_ -> Boolean) (listof X) -> (union X false)
+(define (find-non pred? l)
+  (let ([r (filter (compose not pred?) l)])
+    (if (null? r) #f (car r))))
+
+#| Tests ------------------------------------------------------------------
   (not (find-non list? '((1 2 3) (a b c))))
   (symbol? (find-non number? '(1 2 3 a)))
   (symbol? (find-non list? '((1 2 3) a (b c))))
   |#
 
-  (define-struct (tp-exn exn) ())
-     
-  (define (tp-error name fmt . args)
-    (raise (make-tp-exn (string-append (format "~a: " name) (apply format fmt args))
-                        (current-continuation-marks))))
+(define-struct (tp-exn exn) ())
 
-  (define (number->ord i)
-    (if (= i 0)
-        "zeroth"
-        (case (modulo i 10)
-          [(0 4 5 6 7 8 9) (format "~ath" i)]
-          [(1) (format "~ast" i)]
-          [(2) (format "~and" i)]
-          [(3) (format "~ard" i)])))
-     
-  ;; Symbol (union true String) String X -> void
-  (define (check-list-list pname condition pred given)
-    (when (string? condition)
-      (tp-error pname (string-append condition (format "~nin ~e" given)))))
+(define (tp-error name fmt . args)
+  (raise (make-tp-exn (string-append (format "~a: " name) (apply format fmt args))
+                      (current-continuation-marks))))
 
-  ;; Symbol (_ -> Boolean) String X  X *-> X 
-  (define (check-result pname pred? expected given . other-given)
-    (if (pred? given)
-	given
-	(tp-error pname "result of type <~a> expected, given: ~a" expected 
-                  (if (pair? other-given)
-                      (car other-given)
-                      given))))
+(define (number->ord i)
+  (if (= i 0)
+      "zeroth"
+      (case (modulo i 10)
+        [(0 4 5 6 7 8 9) (format "~ath" i)]
+        [(1) (format "~ast" i)]
+        [(2) (format "~and" i)]
+        [(3) (format "~ard" i)])))
 
-  ;; check-arg : sym bool str str TST -> void
-  (define (check-arg pname condition expected arg-posn given)
-    (unless condition
-      (tp-error pname "expected <~a> as ~a argument, given: ~e"
-	expected arg-posn given)))
-  
-  ;; check-arity : sym num (list-of TST) -> void
-  (define (check-arity name arg# args)
-    (if (= (length args) arg#)
-        (void)
-        (tp-error name "expects ~a arguments, given ~e" arg# (length args))))
-  
-  ;; check-proc :
-  ;;   sym (... *->* ...) num (union sym str) (union sym str) -> void
-  (define (check-proc proc f exp-arity arg# arg-err)
-    (unless (procedure? f)
-      (tp-error proc "procedure expected as ~s argument; given ~e" arg# f))
-    (unless (procedure-arity-includes? f exp-arity)
-      (let ([arity-of-f (procedure-arity f)])
-        (tp-error proc "procedure of ~a expected as ~a argument; given procedure of ~a "
-	  arg-err arg# 
-	  (cond
-	    [(number? arity-of-f)
-	     (if (= arity-of-f 1)
-		 (format "1 argument")
-		 (format "~s arguments" arity-of-f))]
-	    [(arity-at-least? arity-of-f) (format "at least ~s arguments" (arity-at-least-value arity-of-f))]
-	    [else (format "multiple arities (~s)" arity-of-f)]))))))
+;; Symbol (union true String) String X -> void
+(define (check-list-list pname condition pred given)
+  (when (string? condition)
+    (tp-error pname (string-append condition (format "~nin ~e" given)))))
+
+;; Symbol (_ -> Boolean) String X  X *-> X 
+(define (check-result pname pred? expected given . other-given)
+  (if (pred? given)
+      given
+      (tp-error pname "result of type <~a> expected, given: ~a" expected 
+                (if (pair? other-given)
+                    (car other-given)
+                    given))))
+
+;; check-arg : sym bool str str TST -> void
+(define (check-arg pname condition expected arg-posn given)
+  (unless condition
+    (tp-error pname "expected <~a> as ~a argument, given: ~e"
+              expected arg-posn given)))
+
+;; check-arity : sym num (list-of TST) -> void
+(define (check-arity name arg# args)
+  (if (= (length args) arg#)
+      (void)
+      (tp-error name "expects ~a arguments, given ~e" arg# (length args))))
+
+;; check-proc :
+;;   sym (... *->* ...) num (union sym str) (union sym str) -> void
+(define (check-proc proc f exp-arity arg# arg-err)
+  (unless (procedure? f)
+    (tp-error proc "procedure expected as ~s argument; given ~e" arg# f))
+  (unless (procedure-arity-includes? f exp-arity)
+    (let ([arity-of-f (procedure-arity f)])
+      (tp-error proc "procedure of ~a expected as ~a argument; given procedure of ~a "
+                arg-err arg# 
+                (cond
+                  [(number? arity-of-f)
+                   (if (= arity-of-f 1)
+                       (format "1 argument")
+                       (format "~s arguments" arity-of-f))]
+                  [(arity-at-least? arity-of-f) (format "at least ~s arguments" (arity-at-least-value arity-of-f))]
+                  [else (format "multiple arities (~s)" arity-of-f)])))))