diff --git a/collects/drracket/private/drsig.rkt b/collects/drracket/private/drsig.rkt
index 0a700f6a20..06291cc64a 100644
--- a/collects/drracket/private/drsig.rkt
+++ b/collects/drracket/private/drsig.rkt
@@ -190,7 +190,10 @@
    get-definitions-text
    get-interactions-canvas
    get-definitions-canvas
-   get-unit-frame))
+   get-unit-frame
+   
+   allow-re-extension!
+   disallow-re-extension!))
 
 (define-signature drracket:unit-cm^
   (tab%
diff --git a/collects/drracket/private/get-extend.rkt b/collects/drracket/private/get-extend.rkt
index 63a3c82903..165c5a1010 100644
--- a/collects/drracket/private/get-extend.rkt
+++ b/collects/drracket/private/get-extend.rkt
@@ -13,6 +13,10 @@
         [prefix drracket:module-language-tools: drracket:module-language-tools^])
 (export drracket:get/extend^)
 
+(define re-extension-allowed? #f)
+(define (allow-re-extension!) (set! re-extension-allowed? #t))
+(define (disallow-re-extension!) (set! re-extension-allowed? #f))
+
 (define make-extender
   (λ (get-base% name [final-mixin values])
     (let ([extensions (λ (x) x)]
@@ -27,22 +31,26 @@
                      new%
                      (error 'extend-% "expected output of extension to create a subclass of its input, got: ~a"
                             new%)))))])
+      (define (add-extender extension [before? #t])
+        (when built-yet?
+          (cond
+            [re-extension-allowed? 
+             (set! built-yet? #f)]
+            [else
+             (error 'extender "cannot build a new extension of ~a after initialization"
+                    name)]))
+        (set! extensions 
+              (if before?
+                  (compose (verify extension) extensions)
+                  (compose extensions (verify extension)))))
+      (define (get-built)
+        (unless built-yet?
+          (set! built-yet? #t)
+          (set! built (final-mixin (extensions (get-base%)))))
+        built)
       (values
-       (letrec ([add-extender
-                 (λ (extension [before? #t])
-                   (when built-yet?
-                     (error 'extender "cannot build a new extension of ~a after initialization"
-                            name))
-                   (set! extensions 
-                         (if before?
-                             (compose (verify extension) extensions)
-                             (compose extensions (verify extension)))))])
-         add-extender)
-       (λ ()
-         (unless built-yet?
-           (set! built-yet? #t)
-           (set! built (final-mixin (extensions (get-base%)))))
-         built)))))
+       (procedure-rename add-extender (string->symbol (format "extend-~a" name)))
+       (procedure-rename get-built (string->symbol (format "get-~a" name)))))))
 
 (define (get-base-tab%)
   (drracket:module-language:module-language-online-expand-tab-mixin
diff --git a/collects/drracket/tool-lib.rkt b/collects/drracket/tool-lib.rkt
index f5f4d0ff40..a5cb650e07 100644
--- a/collects/drracket/tool-lib.rkt
+++ b/collects/drracket/tool-lib.rkt
@@ -969,6 +969,7 @@ all of the names in the tools library, for use defining keybindings
  ;   ;;;;                                                                 
  
  
+ 
  (proc-doc/names
   drracket:get/extend:extend-tab
   (->* ((make-mixin-contract drracket:unit:tab<%>))
@@ -1002,7 +1003,9 @@ all of the names in the tools library, for use defining keybindings
   
   @{Once this function is called, 
     @racket[drracket:get/extend:extend-interactions-text] 
-    raises an error, disallowing any more extensions.})
+    raises an error, disallowing any more extensions.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:extend-definitions-text
@@ -1014,7 +1017,9 @@ all of the names in the tools library, for use defining keybindings
   @{This text is used in the top window of DrRacket frames.
     
     The @racket[before] argument controls if the mixin is applied before or
-    after already installed mixins.})
+    after already installed mixins.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:get-definitions-text
@@ -1023,7 +1028,9 @@ all of the names in the tools library, for use defining keybindings
   
   @{Once this function is called,
     @racket[drracket:get/extend:extend-definitions-text] 
-    raises an error, disallowing any more extensions.})
+    raises an error, disallowing any more extensions.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:extend-interactions-canvas
@@ -1044,7 +1051,9 @@ all of the names in the tools library, for use defining keybindings
   
   @{Once this function is called, 
     @racket[drracket:get/extend:extend-interactions-canvas]
-    raises an error, disallowing any more extensions.})
+    raises an error, disallowing any more extensions.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:extend-definitions-canvas
@@ -1056,7 +1065,9 @@ all of the names in the tools library, for use defining keybindings
   @{This canvas is used in the top window of DrRacket frames.
     
     The @racket[before] argument controls if the mixin is applied before or
-    after already installed mixins.})
+    after already installed mixins.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:get-definitions-canvas
@@ -1065,7 +1076,9 @@ all of the names in the tools library, for use defining keybindings
   
   @{Once this function is called, 
     @racket[drracket:get/extend:extend-definitions-canvas]
-    raises an error, disallowing any more extensions.})
+    raises an error, disallowing any more extensions.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:extend-unit-frame
@@ -1077,7 +1090,9 @@ all of the names in the tools library, for use defining keybindings
   @{This is the frame that implements the main DrRacket window.
     
     The argument, @racket[before], controls if the mixin is applied before or
-    after already installed mixins.})
+    after already installed mixins.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
  (proc-doc/names
   drracket:get/extend:get-unit-frame
@@ -1086,9 +1101,24 @@ all of the names in the tools library, for use defining keybindings
   
   @{Once this function is called, 
     @racket[drracket:get/extend:extend-unit-frame]
-    raises an error, disallowing any more extensions.})
+    raises an error, disallowing any more extensions.
+    
+    See also @racket[drracket:get/extend:allow-re-extension!].})
  
+ (proc-doc/names
+  drracket:get/extend:disallow-re-extension!
+  (-> void?)
+  ()
+  @{Once this is called, re-extension of the mixins described in this
+    section is not allowed. This is the default state of mixin extension,
+    but it can be changed by @racket[drracket:get/extend:allow-re-extension!].})
  
+ (proc-doc/names
+  drracket:get/extend:allow-re-extension!
+  (-> void?)
+  ()
+  @{Once this is called, re-extension of the mixins described in this
+    section are now allowed.})
  
  ;                                                
  ;