diff --git a/collects/2htdp/private/check-aux.rkt b/collects/2htdp/private/check-aux.rkt
index 896ae61614..45b8d96cb7 100644
--- a/collects/2htdp/private/check-aux.rkt
+++ b/collects/2htdp/private/check-aux.rkt
@@ -10,10 +10,6 @@
 (define PAUSE  1/2)     ;; # secs to wait between attempts to connect to server 
 (define SQPORT 4567) ;; the port on which universe traffic flows
 
-(define (K w . r) w)
-(define (False w) #f)
-(define (True w) #t)
-
 ;                                                                               
 ;                                                                               
 ;                                                                               
@@ -32,6 +28,10 @@
 
 ;; -----------------------------------------------------------------------------
 
+;; Any -> Boolean
+(define (nat? x)
+  (and (number? x) (integer? x) (>= x 0)))
+
 ;; Number Symbol Symbol -> Integer
 (define (number->integer x [t ""] [p ""])
   (check-arg t (and (number? x) (real? x)) "real number" p x)
diff --git a/collects/2htdp/private/syn-aux.rkt b/collects/2htdp/private/clauses-spec-and-process.rkt
similarity index 62%
rename from collects/2htdp/private/syn-aux.rkt
rename to collects/2htdp/private/clauses-spec-and-process.rkt
index c70e1a92fc..36b85595bd 100644
--- a/collects/2htdp/private/syn-aux.rkt
+++ b/collects/2htdp/private/clauses-spec-and-process.rkt
@@ -1,62 +1,63 @@
 #lang racket
 
-(provide define-keywords 
-         DEFAULT
-         ;; constraint: the first kw is the original one 
+;; ---------------------------------------------------------------------------------------------------
+;; provides functions for specifying the shape of big-bang and universe clauses:
+
+(provide function-with-arity expr-with-check except err)
+
+;; ... and for checking and processing them 
+
+(provide ;; constraint: the first kw is the original one 
          ;; and it is also the name of the field in the class
          ->args
-         function-with-arity expr-with-check except err 
-         ->kwds-in
-         clauses-use-kwd
          contains-clause?)
 
 (require 
- (for-syntax "syn-aux-aux.rkt" syntax/parse)
- (for-template "syn-aux-aux.rkt"
-               racket
-               (rename-in lang/prim (first-order->higher-order f2h))))
+ (for-syntax syntax/parse)
+ (for-template "clauses-spec-aux.rkt" racket (rename-in lang/prim (first-order->higher-order f2h))))
 
-(define-syntax (DEFAULT stx)
-  (raise-syntax-error 'DEFAULT "used out of context" stx))
+;; ---------------------------------------------------------------------------------------------------
+;; specifying the shape of clauses 
 
-(define-syntax (define-keywords stx)
-  (syntax-parse stx #:literals (DEFAULT) 
-    [(_ the-list super-list define-create
-        (kw:identifier 
-         (~optional kw-alt:identifier #:defaults ((kw-alt #'kw)))
-         (~optional (~seq DEFAULT default:expr))
-         coerce:expr) ...)
-     (let* ([defs (attribute default)])
-       #`(begin
-           ;; define and create list of keywords and associated values 
-           (define-for-syntax the-list
-             (list* (list #'kw #'kw-alt (coerce ''kw) default) ... super-list))
-           ;; define and provide keywords
-           (provide (rename-out (kw  kw-alt) ...))
-           (provide kw ...) 
-           (define-syntaxes (kw ...)
-             (values (lambda (x)
-                       (raise-syntax-error 'kw "used out of context" x))
-                     ...))
-           
-           ;; a macro for creating functions that instantiate the proper object
-           ;; (define-create para ...) :: additional parameters for the new func
-           (define-syntax (define-create stx)
-             (syntax-case stx ()
-               [(_ para (... ...))
-                (let*-values
-                    ([(kwds defs)
-                      (values (map car the-list) '())]
-                     ;; the defaults list defs is no longer needed
-                     [(args) (lambda (para*)
-                               (append para* (foldr cons '() kwds)))]
-                     [(body) (lambda (para*)
-                               (map (lambda (x) `(,x ,x)) (append para* kwds)))])
-                  (let ([para* (syntax->list #'(para (... ...)))])
-                    #`(lambda (%)
-                        (lambda #,(args para*)
-                          (lambda ()
-                            (new % #,@(body para*)))))))]))))]))
+(define-syntax (expr-with-check stx)
+  (syntax-case stx ()
+    [(_ check> msg)
+     #`(lambda (tag)
+         (lambda (p)
+           (syntax-case p ()
+             [(_ x) #`(check> #,tag x)]
+             [_ (err tag p msg)])))]))
+
+(define-syntax function-with-arity 
+  (syntax-rules (except)
+    [(_ arity)
+     (lambda (tag)
+       (lambda (p)
+         (syntax-case p ()
+           [(_ x) #`(proc> #,tag (f2h x) arity)]
+           [_ (err tag p)])))]
+    [(_ arity except extra)
+     (lambda (tag)
+       (lambda (p)
+         (syntax-case p ()
+           [(_ x) #`(proc> #,tag (f2h x) arity)]
+           extra
+           [_ (err tag p)])))]))
+
+(define (err spec p . xtras)
+  (raise-syntax-error (cadr spec)
+                      (if (null? xtras)
+                          "illegal specification"
+                          (string-append "illegal specification: " (car xtras)))
+                      p))
+
+;; ---------------------------------------------------------------------------------------------------
+;; processing actual clauses 
+
+;; KeyWord [Listof Clause] -> Boolean 
+;; does this list of clauses contain one that starts with kw?
+(define (contains-clause? kw clause-list)
+  (memf (lambda (clause) (free-identifier=? kw (car (syntax->list clause)))) clause-list))
 
 #|
   transform the clauses into the initial arguments specification 
@@ -91,16 +92,25 @@
          (if r ((third s) r) (fourth s)))
        Spec))
 
-(define (contains-clause? kw clause-list)
-  (memf (lambda (clause) (free-identifier=? kw (car (syntax->list clause)))) clause-list))
+;; check whether rec? occurs, produce list of keyword x clause pairs 
+(define (clauses-use-kwd stx:list ->rec? legal-clause kwds)
+  (define kwd-in? (->kwds-in kwds))
+  (define double (string-append legal-clause ", ~a has been redefined"))
+  (map (lambda (stx)
+         (syntax-case stx ()
+           [(kw . E) (kwd-in? #'kw) (begin (->rec? #'kw #'E) (cons #'kw stx))]
+           [(kw . E)
+            (let ([kw (syntax-e #'kw)])
+              (if (member kw (map syntax-e kwds))
+                  (raise-syntax-error #f (format double kw) stx)
+                  (raise-syntax-error #f legal-clause stx)))]
+           [_ (raise-syntax-error #f legal-clause stx)]))
+       stx:list))
 
-
-;; Syntax -> Syntax 
-;; eventually: convert syntax to keyword
-(define (mk-kwd key)
-  (define key:id (symbol->string (syntax-e key)))
-  (define key:wd (string->keyword key:id))
-  key:wd)
+;; [Listof SyntaxIdentifier] -> (Syntax -> Boolean)
+(define (->kwds-in kwds)
+  (lambda (k)
+    (and (identifier? k) (for/or ([n kwds]) (free-identifier=? k n)))))
 
 ;; Symbol Syntax Syntax [Listof Kw] -> true
 ;; effect: if state0 looks like a clause, raise special error 
@@ -124,55 +134,3 @@
          (if x 
              (raise-syntax-error tag (format "duplicate ~a clause" id) (cdar x))
              (duplicates? (rest lox))))])))
-
-;; check whether rec? occurs, produce list of keyword x clause pairs 
-(define (clauses-use-kwd stx:list ->rec? legal-clause kwds)
-  (define kwd-in? (->kwds-in kwds))
-  (define double (string-append legal-clause ", ~a has been redefined"))
-  (map (lambda (stx)
-         (syntax-case stx ()
-           [(kw . E) (kwd-in? #'kw) (begin (->rec? #'kw #'E) (cons #'kw stx))]
-           [(kw . E)
-            (let ([kw (syntax-e #'kw)])
-              (if (member kw (map syntax-e kwds))
-                  (raise-syntax-error #f (format double kw) stx)
-                  (raise-syntax-error #f legal-clause stx)))]
-           [_ (raise-syntax-error #f legal-clause stx)]))
-       stx:list))
-
-;; [Listof SyntaxIdentifier] -> (Syntax -> Boolean)
-(define (->kwds-in kwds)
-  (lambda (k)
-    (and (identifier? k) (for/or ([n kwds]) (free-identifier=? k n)))))
-
-(define-syntax (expr-with-check stx)
-  (syntax-case stx ()
-    [(_ check> msg)
-     #`(lambda (tag)
-         (lambda (p)
-           (syntax-case p ()
-             [(_ x) #`(check> #,tag x)]
-             [_ (err tag p msg)])))]))
-
-(define-syntax function-with-arity 
-  (syntax-rules (except)
-    [(_ arity)
-     (lambda (tag)
-       (lambda (p)
-         (syntax-case p ()
-           [(_ x) #`(proc> #,tag (f2h x) arity)]
-           [_ (err tag p)])))]
-    [(_ arity except extra)
-     (lambda (tag)
-       (lambda (p)
-         (syntax-case p ()
-           [(_ x) #`(proc> #,tag (f2h x) arity)]
-           extra
-           [_ (err tag p)])))]))
-
-(define (err spec p . xtras)
-  (raise-syntax-error (cadr spec)
-                      (if (null? xtras)
-                          "illegal specification"
-                          (string-append "illegal specification: " (car xtras)))
-                      p))
diff --git a/collects/2htdp/private/clauses-spec-aux.rkt b/collects/2htdp/private/clauses-spec-aux.rkt
new file mode 100644
index 0000000000..039b881975
--- /dev/null
+++ b/collects/2htdp/private/clauses-spec-aux.rkt
@@ -0,0 +1,48 @@
+#lang racket
+
+;; ---------------------------------------------------------------------------------------------------
+;; provides constants and functions for specifying the shape of clauses in big-bang and universe 
+
+(provide nat> nat? proc> bool> num> ip> string> symbol> any> K False True)
+
+(require htdp/error "check-aux.rkt")
+                             
+(define (K w . r) w)
+(define (False w) #f)
+(define (True w) #t)
+
+;; Symbol X -> X
+(define (bool> tag x)
+  (check-arg tag (boolean? x) "boolean" "first" x)
+  x)
+
+;; Symbol X -> X
+(define (string> tag x)
+  (check-arg tag (string? x) "string" "first" x)
+  x)
+
+(define ip> string>)
+
+;; Symbol X -> X
+(define (symbol> tag x)
+  (check-arg tag (symbol? x) "symbol" "second" x)
+  x)
+
+;; Symbol X Nat -> X
+(define (proc> tag f ar)
+  (check-proc tag f ar "first" (if (> ar 1) (format "~a arguments" ar) "one argument"))
+  f)
+
+;; Symbol X (Number -> Boolean) String String -> X
+(define (num> tag x pred? spec which)
+  (check-arg tag (and (number? x) (pred? x)) spec which x)
+  x)
+
+;; Symbol X String -> X
+(define (nat> tag x spec)
+  (check-arg tag (nat? x) spec "natural number" x)
+  x)
+
+;; Symbol X String -> X
+(define (any> tag x)
+  x)
diff --git a/collects/2htdp/private/define-keywords.rkt b/collects/2htdp/private/define-keywords.rkt
new file mode 100644
index 0000000000..7a59aa2da8
--- /dev/null
+++ b/collects/2htdp/private/define-keywords.rkt
@@ -0,0 +1,47 @@
+#lang racket
+
+;; ---------------------------------------------------------------------------------------------------
+;; provide a mechanism for defining the shape of big-bang and universe clauses 
+;; to specify individual clauses see syn-aux.rkt
+
+(provide define-keywords DEFAULT)
+
+(require (for-syntax syntax/parse))
+
+(define-syntax (DEFAULT stx)
+  (raise-syntax-error 'DEFAULT "used out of context" stx))
+
+(define-syntax (define-keywords stx)
+  (syntax-parse stx #:literals (DEFAULT) 
+    [(_ the-list super-list define-create
+        (kw:identifier 
+         (~optional kw-alt:identifier #:defaults ((kw-alt #'kw)))
+         (~optional (~seq DEFAULT default:expr))
+         coerce:expr) ...)
+     (let* ([defs (attribute default)])
+       #`(begin
+           ;; define and create list of keywords and associated values 
+           (define-for-syntax the-list
+             (list* (list #'kw #'kw-alt (coerce ''kw) default) ... super-list))
+           ;; define and provide keywords
+           (provide (rename-out (kw  kw-alt) ...))
+           (provide kw ...) 
+           (define-syntaxes (kw ...)
+             (values (lambda (x) (raise-syntax-error 'kw "used out of context" x)) ...))
+           
+           ;; a macro for creating functions that instantiate the proper object
+           ;; (define-create para ...) :: additional parameters for the new func
+           (define-syntax (define-create stx)
+             (syntax-case stx ()
+               [(_ para (... ...))
+                (let*-values
+                    ([(kwds defs)
+                      (values (map car the-list) '())]
+                     ;; the defaults list defs is no longer needed
+                     [(args) (lambda (para*) (append para* (foldr cons '() kwds)))]
+                     [(body) (lambda (para*) (map (lambda (x) `(,x ,x)) (append para* kwds)))])
+                  (let ([para* (syntax->list #'(para (... ...)))])
+                    #`(lambda (%)
+                        (lambda #,(args para*)
+                          (lambda ()
+                            (new % #,@(body para*)))))))]))))]))
diff --git a/collects/2htdp/private/design.txt b/collects/2htdp/private/design.txt
index df68373fc0..838f645f01 100644
--- a/collects/2htdp/private/design.txt
+++ b/collects/2htdp/private/design.txt
@@ -1,17 +1,17 @@
 
-Files for constructing universe.ss: 
+Files for constructing universe.rkt: 
 
- world.ss              the old world 
- 		       world%  = (clock-mixin ...) -- the basic world 
-		       aworld% = (class world% ...) -- the world with recording
+ world.rkt              the old world 
+ 		         world%  = (clock-mixin ...) -- the basic world 
+		         aworld% = (class world% ...) -- the world with recording
 
- universe.ss	       the universe server 
- 		       universe% = (clock-mixin ...) -- the basic universe
+ universe.rkt	        the universe server 
+ 		        universe% = (clock-mixin ...) -- the basic universe
 
- timer.ss              the clock-mixin
+ timer.rkt              the clock-mixin
 
- check-aux.ss          common primitives 
- image.ss              the world image functions 
- syn-aux.ss            syntactic auxiliaries 
- syn-aux-aux.ss        auxiliaries to the syntactic auxiliaries 
+ check-aux.rkt          common primitives 
+ image.rkt              the world image functions 
+ clauses-spec-and-process.rkt syntactic auxiliaries 
+ clauses-spec-aux.rkt        auxiliaries to the syntactic auxiliaries 
 
diff --git a/collects/2htdp/private/syn-aux-aux.rkt b/collects/2htdp/private/syn-aux-aux.rkt
deleted file mode 100644
index fbd8face8a..0000000000
--- a/collects/2htdp/private/syn-aux-aux.rkt
+++ /dev/null
@@ -1,64 +0,0 @@
-#lang racket
-
-(require htdp/error "check-aux.rkt")
-
-;                                                   
-;                                                   
-;                                                   
-;    ;;;                         ;;;                
-;   ;   ;                       ;   ;               
-;   ;                           ;   ;               
-;   ;      ;   ;  ;;;;          ;   ;  ;   ;  ;   ; 
-;    ;;;   ;   ;  ;   ;         ;;;;;  ;   ;   ; ;  
-;       ;  ;   ;  ;   ;         ;   ;  ;   ;    ;   
-;       ;  ;   ;  ;   ;         ;   ;  ;   ;    ;   
-;   ;   ;  ;   ;  ;   ;         ;   ;  ;  ;;   ; ;  
-;    ;;;    ;;;;  ;   ;         ;   ;   ;; ;  ;   ; 
-;              ;                                    
-;          ;   ;                                    
-;           ;;;                                     
-
-(provide nat> nat? proc> bool> num> ip> string> symbol> any>)
-
-;; Any -> Boolean
-(define (nat? x)
-  (and (number? x) (integer? x) (>= x 0)))
-
-;; Symbol X -> X
-(define (bool> tag x)
-  (check-arg tag (boolean? x) "boolean" "first" x)
-  x)
-
-;; Symbol X -> X
-(define (string> tag x)
-  (check-arg tag (string? x) "string" "first" x)
-  x)
-
-(define ip> string>)
-
-;; Symbol X -> X
-(define (symbol> tag x)
-  (check-arg tag (symbol? x) "symbol" "second" x)
-  x)
-
-;; Symbol X Nat -> X
-(define (proc> tag f ar)
-  (check-proc tag f ar "first" 
-              (if (> ar 1)
-                  (format "~a arguments" ar)
-                  "one argument"))
-  f)
-
-;; Symbol X (Number -> Boolean) String String -> X
-(define (num> tag x pred? spec which)
-  (check-arg tag (and (number? x) (pred? x)) spec which x)
-  x)
-
-;; Symbol X String -> X
-(define (nat> tag x spec)
-  (check-arg tag (nat? x) spec "natural number" x)
-  x)
-
-;; Symbol X String -> X
-(define (any> tag x)
-  x)
diff --git a/collects/2htdp/private/universe.rkt b/collects/2htdp/private/universe.rkt
index 691a15fd96..ebf6190eaa 100644
--- a/collects/2htdp/private/universe.rkt
+++ b/collects/2htdp/private/universe.rkt
@@ -1,15 +1,34 @@
 #lang racket/gui
 
-(require (for-syntax "syn-aux.rkt")
-         "checked-cell.rkt"
+;; ---------------------------------------------------------------------------------------------------
+;; provides the universe functionality (distributed worlds)
+
+(require "checked-cell.rkt"
          "check-aux.rkt"
-         "timer.rkt"
+         "timer.rkt"    
          "last.rkt"
+         "clauses-spec-aux.rkt"
          htdp/error
          (only-in mzlib/etc evcase)
          string-constants)
 
-(provide universe%)
+(provide 
+ universe%
+ ;; --- sample worlds and function on worlds ---
+ iworld?  ;; Any -> Boolean 
+ iworld=? ;; World World -> Boolean 
+ iworld-name ;; World -> Symbol 
+ iworld1  ;; sample worlds 
+ iworld2
+ iworld3
+ ;; --- sending 'mail' to worlds ---
+ ;; type Bundle = (make-bundle [Listof World] Universe [Listof Mail]) 
+ ;; type Mail = (make-mail World S-expression)
+ make-bundle ;; [Listof World] Universe [Listof Mail] -> Bundle 
+ bundle?     ;; is this a bundle? 
+ make-mail   ;; World S-expression -> Mail 
+ mail?       ;; is this a real mail? 
+ )
 
 ;                                                          
 ;                                                          
@@ -205,14 +224,6 @@
 ;                                            
 ;                                            
 
-(provide 
- iworld?  ;; Any -> Boolean 
- iworld=? ;; World World -> Boolean 
- iworld-name ;; World -> Symbol 
- iworld1  ;; sample worlds 
- iworld2
- iworld3)
-
 ;; --- the server representation of a world --- 
 (define-struct iworld (in out name info) #; #:transparent)
 ;; World = (make-iworld IPort OPort Symbol [Listof Sexp])
@@ -332,15 +343,6 @@
 ;                              
 ;                              
 
-(provide
- ;; type Bundle = (make-bundle [Listof World] Universe [Listof Mail]) 
- ;; type Mail = (make-mail World S-expression)
- make-bundle ;; [Listof World] Universe [Listof Mail] -> Bundle 
- bundle?     ;; is this a bundle? 
- make-mail   ;; World S-expression -> Mail 
- mail?       ;; is this a real mail? 
- )
-
 (define-struct bundle (state mails bad) #:transparent)
 
 (set! make-bundle
diff --git a/collects/2htdp/tests/record-stop-when.rkt b/collects/2htdp/tests/record-stop-when.rkt
index 2c5ba4d9eb..130cfa30b5 100644
--- a/collects/2htdp/tests/record-stop-when.rkt
+++ b/collects/2htdp/tests/record-stop-when.rkt
@@ -26,7 +26,11 @@
             (on-draw draw-number)
             (record? dir)))
 (sleep 1)
-(unless (image=? (bitmap "images0/i1.png") (draw-stop 5))
+
+(define i (bitmap "images0/i1.png"))
+(define j (draw-stop 5))
+
+(unless (image=? (crop 0 0 100 100 i) j)
   (fprintf (current-error-port)
            "this test needs to be revised -- the way 'world' writes images adds an extra pixel -- think! \n"))
 
diff --git a/collects/2htdp/universe.rkt b/collects/2htdp/universe.rkt
index 2ead67ef91..1b0e61c7d5 100644
--- a/collects/2htdp/universe.rkt
+++ b/collects/2htdp/universe.rkt
@@ -1,10 +1,11 @@
 #lang racket/gui
 
-;; DONT USE to-draw IN THIS FILE 
+;; ---------------------------------------------------------------------------------------------------
+;; the universe library provides the functionality to create interactive and distributed FPs in HtDP
+
+;; DONT USE ___to-draw___ IN THIS FILE 
 
 #| TODO: 
-   -- check that on-release is only defined if on-key is defined 
-
    -- run callbacks in user eventspace
    -- make timer fire just once; restart after on-tick callback finishes
    -- take out counting; replace by 0.25 delay
@@ -12,16 +13,18 @@
    -- make window resizable :: why
 |#
 
-(require (for-syntax "private/syn-aux.rkt"
+(require (for-syntax "private/clauses-spec-and-process.rkt"
                      stepper/private/shared)
-         "private/syn-aux-aux.rkt"
-         "private/syn-aux.rkt"
-         "private/check-aux.rkt"
-         "private/universe-image.rkt"
+         "private/define-keywords.rkt"
+         "private/clauses-spec-aux.rkt" 
+         ;; ---
          "private/world.rkt"
          "private/universe.rkt"
-         "private/launch-many-worlds.rkt"
-         "private/stop.rkt"
+         "private/universe-image.rkt"
+         ;; 
+         (only-in "private/launch-many-worlds.rkt" launch-many-worlds)
+         (only-in "private/stop.rkt" make-stop-the-world)
+         (only-in "private/check-aux.rkt" sexp?)
          htdp/error
          (rename-in lang/prim (first-order->higher-order f2h)))
 
@@ -314,8 +317,7 @@
     [(universe) (raise-syntax-error #f "not a legal universe description" stx)]
     [(universe u) (raise-syntax-error #f "not a legal universe description" stx)]
     [(universe u bind ...)
-     (let* ([args
-             (->args 'universe stx #'u #'(bind ...) UniSpec void "universe")]
+     (let* ([args (->args 'universe stx #'u #'(bind ...) UniSpec void "universe")]
             [dom (syntax->list #'(bind ...))])
        (cond
          [(not (contains-clause? #'on-new dom))