diff --git a/collects/unstable/list.rkt b/collects/unstable/list.rkt
index a7440692f9..206a8c58bf 100644
--- a/collects/unstable/list.rkt
+++ b/collects/unstable/list.rkt
@@ -33,7 +33,7 @@
     prefix))
 
 (define (drop-common-prefix as bs #:same? [same? equal?])
-  (let-values ([(atail btail) (internal-split-common-prefix as bs same? #f)])
+  (let-values ([(prefix atail btail) (internal-split-common-prefix as bs same? #f)])
     (values atail btail)))
 
 (provide/contract
diff --git a/collects/unstable/scribblings/class-iop.scrbl b/collects/unstable/scribblings/class-iop.scrbl
index 2af6c20d4f..372d7888aa 100644
--- a/collects/unstable/scribblings/class-iop.scrbl
+++ b/collects/unstable/scribblings/class-iop.scrbl
@@ -9,7 +9,7 @@
 @title[#:tag "class-iop"]{Interface-Oriented Programming for Classes}
 
 @(define the-eval (make-base-eval))
-@(the-eval '(require racket/class unstable/class-iop))
+@(the-eval '(require racket/class unstable/class-iop (for-syntax racket/base)))
 
 @defmodule[unstable/class-iop]
 
@@ -127,4 +127,18 @@ should be fixed.
 
 }
 
+@defform[(define-interface-expander id transformer-expr)]{
+
+Defines @racket[id] as a macro that can be used within
+@racket[define-interface] forms.
+
+@examples[#:eval the-eval
+(define-interface-expander stack-methods
+  (lambda (stx) #'[empty? push pop]))
+(define-interface stack<%> ()
+  ((stack-methods)))
+(interface->method-names stack<%>)
+]
+}
+
 @close-eval[the-eval]
diff --git a/collects/unstable/scribblings/list.scrbl b/collects/unstable/scribblings/list.scrbl
index ac8a423c40..c7378148b2 100644
--- a/collects/unstable/scribblings/list.scrbl
+++ b/collects/unstable/scribblings/list.scrbl
@@ -27,6 +27,41 @@
 ]
 }
 
+@defproc[(take-common-prefix [l list?] [r list?]
+                             [#:same? same? equal?])
+         list?]{
+
+  Returns the longest common prefix of @racket[l] and @racket[r].
+
+@examples[#:eval the-eval
+(take-common-prefix '(a b c d) '(a b x y z))
+]
+}
+
+@defproc[(drop-common-prefix [l list?] [r list?]
+                             [#:same same? equal?])
+         (values list? list?)]{
+
+  Returns the tails of @racket[l] and @racket[r] with the common
+  prefix removed.
+
+@examples[#:eval the-eval
+(drop-common-prefix '(a b c d) '(a b x y z))
+]
+}
+
+@defproc[(split-common-prefix [l list?] [r list?]
+                              [#:same? same? equal?])
+         (values list? list? list?)]{
+
+  Returns the longest common prefix together with the tails of
+  @racket[l] and @racket[r] with the common prefix removed.
+
+@examples[#:eval the-eval
+(split-common-prefix '(a b c d) '(a b x y z))
+]
+}
+
 @addition{Sam Tobin-Hochstadt}
 
 @defproc[(filter-multiple [l list?] [f procedure?] ...) (values list? ...)]{