diff --git a/collects/typed-scheme/main.rkt b/collects/typed-scheme/main.rkt
index 2082ac80..016cdebf 100644
--- a/collects/typed-scheme/main.rkt
+++ b/collects/typed-scheme/main.rkt
@@ -14,4 +14,4 @@
          (for-syntax "private/base-types-extra.rkt"))
 (provide (rename-out [with-handlers: with-handlers])
          (for-syntax (all-from-out "private/base-types-extra.rkt"))
-	 assert with-type for for*)
+	 assert defined? with-type for for*)
diff --git a/collects/typed-scheme/private/base-env.rkt b/collects/typed-scheme/private/base-env.rkt
index a8e248cd..4eccedf0 100644
--- a/collects/typed-scheme/private/base-env.rkt
+++ b/collects/typed-scheme/private/base-env.rkt
@@ -186,6 +186,7 @@
 [assert (-poly (a b) (cl->*
 		      (Univ (make-pred-ty (list a) Univ b) . -> . b)
 		      (-> (Un a (-val #f)) a)))]
+[defined? (->* (list Univ) -Boolean : (-FS (-not-filter -Undefined 0 null) (-filter -Undefined 0 null)))]
 [gensym (->opt [Sym] Sym)]
 [string-append (->* null -String -String)]
 [open-input-string (-> -String -Input-Port)]
diff --git a/collects/typed-scheme/private/extra-procs.rkt b/collects/typed-scheme/private/extra-procs.rkt
index b4a74b03..5b90512b 100644
--- a/collects/typed-scheme/private/extra-procs.rkt
+++ b/collects/typed-scheme/private/extra-procs.rkt
@@ -1,5 +1,5 @@
 #lang scheme/base
-(provide assert)
+(provide assert defined?)
 
 (define-syntax assert
   (syntax-rules ()
@@ -8,3 +8,6 @@
     ((assert v pred)
      (let ((val v))
        (if (pred val) val (error "Assertion failed"))))))
+
+(define (defined? v)
+  (not (equal? v (letrec ([x x]) x))))
\ No newline at end of file
diff --git a/collects/typed/racket/base.rkt b/collects/typed/racket/base.rkt
index 9d0942a2..65d54324 100644
--- a/collects/typed/racket/base.rkt
+++ b/collects/typed/racket/base.rkt
@@ -14,5 +14,5 @@
 	 (for-syntax typed-scheme/private/base-types-extra))
 (provide (rename-out [with-handlers: with-handlers]
                      [define-type-alias define-type])
-	 assert with-type for for*
+	 assert defined? with-type for for*
          (for-syntax (all-from-out typed-scheme/private/base-types-extra)))
diff --git a/collects/typed/scheme/base.rkt b/collects/typed/scheme/base.rkt
index abbf0ebf..4c184c21 100644
--- a/collects/typed/scheme/base.rkt
+++ b/collects/typed/scheme/base.rkt
@@ -14,5 +14,5 @@
 	 (for-syntax typed-scheme/private/base-types-extra))
 (provide (rename-out [with-handlers: with-handlers]
                      [define-type-alias define-type])
-	 assert with-type for for*
+	 assert defined? with-type for for*
          (for-syntax (all-from-out typed-scheme/private/base-types-extra)))