diff --git a/collects/tests/typed-racket/unit-tests/typecheck-tests.rkt b/collects/tests/typed-racket/unit-tests/typecheck-tests.rkt
index ab6754e3..e77d18ce 100644
--- a/collects/tests/typed-racket/unit-tests/typecheck-tests.rkt
+++ b/collects/tests/typed-racket/unit-tests/typecheck-tests.rkt
@@ -339,7 +339,10 @@
          -String]
         [tc-e (string-join '("hello" "world") " ") -String]
         [tc-e (string-join '("hello" "world")) -String]
-        [tc-e (string-join '("hello" "world") #:first "a") -String]
+        [tc-e (string-join '("hello" "world") #:before-first "a") -String]
+        [tc-e (add-between '(1 2 3) 0) (-lst -Byte)]
+        [tc-e (add-between '(1 2 3) 'a) (-lst (t:Un -PosByte (-val 'a)))]
+        [tc-e ((inst add-between Positive-Byte Symbol) '(1 2 3) 'a #:before-first 'b) (-lst (t:Un -PosByte -Symbol))]
 
         [tc-e (apply (plambda: (a) [x : a *] x) '(5)) (-lst -PosByte)]
         [tc-e (apply append (list '(1 2 3) '(4 5 6))) (-lst -PosByte)]
diff --git a/collects/typed-racket/base-env/base-env.rkt b/collects/typed-racket/base-env/base-env.rkt
index a63b5b7d..5cd1525e 100644
--- a/collects/typed-racket/base-env/base-env.rkt
+++ b/collects/typed-racket/base-env/base-env.rkt
@@ -366,7 +366,7 @@
 
 [string-join
  (->optkey (-lst -String) [-String]
-           #:before-last -String #f #:first -String #f #:last -String #f
+           #:before-last -String #f #:before-first -String #f #:after-last -String #f
            -String)]
 
 
@@ -1393,7 +1393,13 @@
                      (-> (make-pred-ty a) (-lst b) (-values (list (-lst a) (-lst b))))))]
 
 [last   (-poly (a) ((-lst a) . -> . a))]
-[add-between (-poly (a b) ((-lst a) b . -> . (-lst (Un a b))))]
+[add-between (-poly (a b) ((-lst a) b
+                           #:splice? -Boolean #f
+                           #:nothing Univ #f ; default is gensym
+                           #:before-first b #f
+                           #:before-last b #f
+                           #:after-last b #f
+                           . ->key . (-lst (Un a b))))]
 
 [last-pair (-poly (a) ((-mu x (Un a (-val '()) (-pair a x)))
                        . -> .