diff --git a/collects/typed-scheme/private/resolve-type.ss b/collects/typed-scheme/private/resolve-type.ss
index 28385b80..d68de692 100644
--- a/collects/typed-scheme/private/resolve-type.ss
+++ b/collects/typed-scheme/private/resolve-type.ss
@@ -1,7 +1,7 @@
 #lang scheme/base
 
 (require "type-rep.ss"  "type-name-env.ss" "tc-utils.ss"
-         "type-utils.ss" 
+         "type-utils.ss"
          mzlib/plt-match
          mzlib/trace)
 
diff --git a/collects/typed-scheme/private/type-utils.ss b/collects/typed-scheme/private/type-utils.ss
index c4e1fa84..b82bf64a 100644
--- a/collects/typed-scheme/private/type-utils.ss
+++ b/collects/typed-scheme/private/type-utils.ss
@@ -17,7 +17,8 @@
          tc-result: 
          tc-result-equal? 
          effects-equal?
-         tc-result-t)
+         tc-result-t
+         unfold)
 
 
 ;; substitute : Type Name Type -> Type
@@ -35,7 +36,12 @@
   (foldr (lambda (e acc) (substitute (cadr e) (car e) acc)) t s))
 
 
-
+;; unfold : Type -> Type
+;; must be applied to a Mu
+(define (unfold t)
+  (match t
+    [(Mu: name b) (substitute t name b)]
+    [_ (int-err "unfold: requires Mu type, got ~a" t)]))
 
 (define (instantiate-poly t types)
   (match t