diff --git a/collects/rackunit/private/gui/gvector.rkt b/collects/data/gvector.rkt
similarity index 53%
rename from collects/rackunit/private/gui/gvector.rkt
rename to collects/data/gvector.rkt
index c5eadfedb7..39ff73a7b6 100644
--- a/collects/rackunit/private/gui/gvector.rkt
+++ b/collects/data/gvector.rkt
@@ -1,17 +1,26 @@
 #lang racket/base
-(require racket/contract
-         racket/dict)
+;; written by ryanc
+(require (for-syntax racket/base
+                     unstable/wrapc)
+         racket/contract/base
+         racket/dict
+         racket/vector)
 
 (define (make-gvector* #:capacity [capacity 10])
   (make-gvector (make-vector capacity #f) 0))
 
-(define (check-index who index n)
+(define (check-index who index n set-to-add?)
   (unless (exact-nonnegative-integer? index)
     (raise-type-error who "exact nonnegative integer" index))
   (unless (< index n)
-    (if (zero? n)
-        (error who "index out of range for empty gvector: ~s" index)
-        (error who "index out of range [0,~s]: ~s" (sub1 n) index))))
+    (error who "index out of range ~a~a: ~s"
+           (let ([max-index (if set-to-add? (- n 2) (- n 1))])
+             (cond [(< max-index 0) "(empty)"]
+                   [else (format "[0,~s]" max-index)]))
+           (if set-to-add?
+               (format " or ~s to add" (- n 1))
+               "")
+           index)))
 
 (define ((bad-index-error who index))
   (raise-mismatch-error who "index out of range" index))
@@ -33,7 +42,7 @@
 (define (gvector-remove! gv index)
   (let ([n (gvector-n gv)]
         [v (gvector-vec gv)])
-    (check-index 'gvector-remove! index n)
+    (check-index 'gvector-remove! index n #f)
     (set-gvector-n! gv (sub1 n))
     (vector-copy! v index v (add1 index) n)
     (vector-set! v (sub1 n) #f)))
@@ -51,27 +60,37 @@
           (default)
           default)))
 
+;; gvector-set! with index = |gv| is interpreted as gvector-add!
 (define (gvector-set! gv index item)
-  (check-index 'gvector-set! index (gvector-n gv))
-  (vector-set! (gvector-vec gv) index item))
+  (let ([n (gvector-n gv)])
+    (check-index 'gvector-set! index (add1 n) #t)
+    (if (= index n)
+        (gvector-add! gv item)
+        (vector-set! (gvector-vec gv) index item))))
+
+;; creates a snapshot vector
+(define (gvector->vector gv)
+  (vector-copy (gvector-vec gv) 0 (gvector-n gv)))
 
 ;; Iteration methods
 
+;; A gvector position is represented as an exact-nonnegative-integer.
+
 (define (gvector-iterate-first gv)
   (and (positive? (gvector-n gv)) 0))
 
 (define (gvector-iterate-next gv iter)
-  (check-index 'gvector-iterate-next iter (gvector-n gv))
+  (check-index 'gvector-iterate-next iter (gvector-n gv) #f)
   (let ([n (gvector-n gv)])
     (and (< (add1 iter) n)
          (add1 iter))))
 
 (define (gvector-iterate-key gv iter)
-  (check-index 'gvector-iterate-key iter (gvector-n gv))
+  (check-index 'gvector-iterate-key iter (gvector-n gv) #f)
   iter)
 
 (define (gvector-iterate-value gv iter)
-  (check-index 'gvector-iterate-value iter (gvector-n gv))
+  (check-index 'gvector-iterate-value iter (gvector-n gv) #f)
   (gvector-ref gv iter))
 
 (define (in-gvector gv)
@@ -79,6 +98,28 @@
     (raise-type-error 'in-gvector "gvector" gv))
   (in-dict-values gv))
 
+(define-sequence-syntax in-gvector*
+  (lambda () #'in-vector)
+  (lambda (stx)
+    (syntax-case stx ()
+      [[(var) (in-gv gv-expr)]
+       (with-syntax ([gv-expr-c (wrap-expr/c #'gvector? #'gv-expr #:macro #'in-gv)])
+         (syntax/loc stx
+           [(var)
+            (:do-in ([(gv) gv-expr-c])
+                    (void) ;; outer-check; handled by contract
+                    ([index 0] [vec (gvector-vec gv)] [n (gvector-n gv)]) ;; loop bindings
+                    (< index n) ;; pos-guard
+                    ([(var) (vector-ref vec index)]) ;; inner bindings
+                    #t ;; pre-guard
+                    #t ;; post-guard
+                    ((add1 index) (gvector-vec gv) (gvector-n gv)))]))]
+      [[(var ...) (in-gv gv-expr)]
+       (with-syntax ([gv-expr-c (wrap-expr/c #'gvector? #'gv-expr #:macro #'in-gv)])
+         (syntax/loc stx
+           [(var ...) (in-vector gv-expr-c)]))]
+      [_ #f])))
+
 (define-struct gvector (vec n)
   #:mutable
   #:property prop:dict
@@ -95,10 +136,12 @@
   #:property prop:sequence in-gvector)
 
 (provide/contract
+ [gvector?
+  (-> any/c any)]
  [rename make-gvector* make-gvector
   (->* () (#:capacity exact-positive-integer?) any)]
  [gvector-ref
-  (-> gvector? exact-nonnegative-integer? any)]
+  (->* (gvector? exact-nonnegative-integer?) (any/c) any)]
  [gvector-set!
   (-> gvector? exact-nonnegative-integer? any/c any)]
  [gvector-add!
@@ -107,5 +150,7 @@
   (-> gvector? exact-nonnegative-integer? any)]
  [gvector-count
   (-> gvector? any)]
- [in-gvector
-  (-> gvector? sequence?)])
+ [gvector->vector
+  (-> gvector? vector?)])
+
+(provide (rename-out [in-gvector* in-gvector]))
diff --git a/collects/data/scribblings/data.scrbl b/collects/data/scribblings/data.scrbl
index baffcf6e7e..bd9132d8be 100644
--- a/collects/data/scribblings/data.scrbl
+++ b/collects/data/scribblings/data.scrbl
@@ -18,4 +18,4 @@ This manual documents data structure libraries available in the
 @include-section["queue.scrbl"]
 @include-section["skip-list.scrbl"]
 @include-section["interval-map.scrbl"]
-
+@include-section["gvector.scrbl"]
diff --git a/collects/data/scribblings/gvector.scrbl b/collects/data/scribblings/gvector.scrbl
new file mode 100644
index 0000000000..deab65fe8e
--- /dev/null
+++ b/collects/data/scribblings/gvector.scrbl
@@ -0,0 +1,99 @@
+#lang scribble/manual
+@(require scribble/eval
+          (for-label data/gvector
+                     racket/contract
+                     racket/dict
+                     racket/base))
+
+@title[#:tag "gvector"]{Growable vectors}
+
+@(define the-eval (make-base-eval))
+@(the-eval '(require data/gvector))
+@(the-eval '(require racket/dict))
+
+@defmodule[data/gvector]
+
+@author[@author+email["Ryan Culpepper" "ryanc@racket-lang.org"]]
+
+A growable vector (gvector) is a mutable sequence whose length can
+change over time. A gvector also acts as a dictionary (@racket[dict?]
+from @racketmodname[racket/dict]), where the keys are zero-based
+indexes and the values are the elements of the gvector. A gvector can
+be extended by adding an element to the end, and it can be shrunk by
+removing any element, although removal can take time linear in the
+number of elements in the gvector.
+
+@defproc[(make-gvector [#:capacity capacity exact-positive-integer? 10])
+         gvector?]{
+
+Creates a new empty growable vector (gvector) with an initial capacity
+of @racket[capacity].
+}
+
+@defproc[(gvector? [x any/c]) boolean?]{
+
+Returns @racket[#t] if @racket[x] was created by
+@racket[make-gvector], @racket[#f] otherwise.
+}
+
+@defproc[(gvector-ref [gv gvector?]
+                      [index exact-nonnegative-integer?]
+                      [default any/c (error ....)])
+         any/c]{
+
+Returns the element at index @racket[index], if @racket[index] is less
+than @racket[(gvector-count gv)]. Otherwise, @racket[default] is
+invoked if it is a procedure, returned otherwise.
+}
+
+@defproc[(gvector-add! [gv gvector?]
+                       [value any/c])
+         void?]{
+
+Adds @racket[value] to the end of the gvector @racket[gv].
+}
+
+@defproc[(gvector-set! [gv gvector?]
+                       [index (and/c exact-nonnegative-integer? 
+                                     (</c (+ 1 (gvector-count gv))))]
+                       [value any/c])
+         void?]{
+
+Sets the value at index @racket[index] to be @racket[value]. If
+@racket[index] is @racket[(gvector-count gv)]---that is, one more than
+the greatest used index---the effect is the same as
+@racket[(gvector-add! gv value)].
+}
+
+@defproc[(gvector-remove! [gv gvector?]
+                          [index (and/c exact-nonnegative-integer?
+                                        (</c (gvector-count gv)))])
+         void?]{
+
+Removes the item at @racket[index], shifting items at higher indexes
+down. Takes time proportional to @racket[(- (gvector-count gv)
+index)].
+}
+
+@defproc[(gvector-count [gv gvector?])
+         exact-nonnegative-integer?]{
+
+Returns the number of items in @racket[gv].
+}
+
+@defproc[(gvector->vector [gv gvector?])
+         vector?]{
+
+Returns a vector of length @racket[(gvector-count gv)] containing the
+elements of @racket[gv] in order.
+}
+
+@defproc[(in-gvector [gv gvector?])
+         sequence?]{
+
+Returns a sequence whose elements are the elements of
+@racket[gv]. Mutation of @racket[gv] while the sequence is running
+changes the elements produced by the sequence. To obtain a sequence
+from a snapshot of @racket[gv], use @racket[(in-vector
+(gvector->vector gv))] instead.
+}
diff --git a/collects/rackunit/private/gui/model.rkt b/collects/rackunit/private/gui/model.rkt
index 110efadbdb..74e25ccb48 100644
--- a/collects/rackunit/private/gui/model.rkt
+++ b/collects/rackunit/private/gui/model.rkt
@@ -2,7 +2,7 @@
 (require racket/class
          unstable/class-iop
          racket/list
-         "gvector.rkt"
+         data/gvector
          "../base.rkt"
          "interfaces.rkt"
          "cache-box.rkt")