diff --git a/cl21.asd b/cl21.asd
index c733339..83e0241 100644
--- a/cl21.asd
+++ b/cl21.asd
@@ -30,6 +30,7 @@
                  (:file "asdf" :depends-on ("cl21" "core-subpackages"))
                  (:module "core-subpackages"
                   :pathname "core"
+                  :depends-on ("internal")
                   :components
                   ((:file "types")
                    (:file "condition")
@@ -43,15 +44,17 @@
                    (:file "cons" :depends-on ("generic" "sequence" "array"))
                    (:file "array" :depends-on ("generic"))
                    (:file "string")
-                   (:file "sequence" :depends-on ("generic" "condition" "environment" "util"))
-                   (:file "hash-table" :depends-on ("generic" "condition" "package" "sequence" "util"))
+                   (:file "sequence" :depends-on ("generic" "condition" "environment"))
+                   (:file "hash-table" :depends-on ("generic" "condition" "package" "sequence"))
                    (:file "file")
                    (:file "stream")
                    (:file "generic" :depends-on ("types"))
                    (:file "repl")
                    (:file "readtable" :depends-on ("function" "array" "sequence" "cons"))
-                   (:file "environment")
-                   (:file "util" :depends-on ("environment"))))
+                   (:file "environment")))
+                 (:module "internal"
+                  :components
+                  ((:file "util")))
                  (:module "stdlib"
                   :depends-on ("core")
                   :components
diff --git a/src/core/cons.lisp b/src/core/cons.lisp
index bfaa8f9..e15e2bc 100644
--- a/src/core/cons.lisp
+++ b/src/core/cons.lisp
@@ -66,7 +66,7 @@
                           :coerce)
   (:shadowing-import-from :cl21.core.array
                           :vector)
-  (:import-from :cl21.core.util
+  (:import-from :cl21.internal.util
                 :define-typecase-compiler-macro)
   (:import-from :alexandria
                 :iota
diff --git a/src/core/hash-table.lisp b/src/core/hash-table.lisp
index 56e4dee..d8eba70 100644
--- a/src/core/hash-table.lisp
+++ b/src/core/hash-table.lisp
@@ -27,7 +27,7 @@
                 :iterator-endp)
   (:shadowing-import-from :cl21.core.sequence
                           :map)
-  (:import-from :cl21.core.util
+  (:import-from :cl21.internal.util
                 :define-typecase-compiler-macro)
   (:import-from :alexandria
                 :alist-hash-table
diff --git a/src/core/sequence.lisp b/src/core/sequence.lisp
index 94f7403..f0e94ae 100644
--- a/src/core/sequence.lisp
+++ b/src/core/sequence.lisp
@@ -66,7 +66,7 @@
                 :method-unimplemented-error)
   (:import-from :cl21.core.environment
                 :variable-information)
-  (:import-from :cl21.core.util
+  (:import-from :cl21.internal.util
                 :define-typecase-compiler-macro)
   #+nil
   (:import-from :split-sequence
diff --git a/src/core/util.lisp b/src/internal/util.lisp
similarity index 92%
rename from src/core/util.lisp
rename to src/internal/util.lisp
index ab47ca0..077f072 100644
--- a/src/core/util.lisp
+++ b/src/internal/util.lisp
@@ -1,12 +1,17 @@
 (in-package :cl-user)
-(defpackage cl21.core.util
+(defpackage cl21.internal.util
   (:use :cl)
-  (:import-from :cl21.core.environment
+  (:import-from #+sbcl :sb-cltl2
+                #+openmcl :ccl
+                #+cmu :ext
+                #+allegro :sys
+                #+ecl :si
+                #+abcl :lisp
                 :variable-information)
   (:import-from :alexandria
                 :ensure-list)
   (:export :define-typecase-compiler-macro))
-(in-package :cl21.core.util)
+(in-package :cl21.internal.util)
 
 (defmacro define-typecase-compiler-macro (name lambda-list typecase)
   (flet ((remove-from-lambda-list (target lambda-list)