```common lisp
(defpackage cl21)
(in-package :cl21)

(cl:dolist (package-name '(:cl21.core))
  (cl:let ((package (cl:find-package package-name)))
    (cl:unless package
      (cl:error "Package \"~A\" doesn't exist." package-name))
    (cl:do-external-symbols (symbol package)
      (cl:shadowing-import symbol)
      (cl:export symbol))))

(in-package :cl-user)
(defpackage cl21-user
  (:use :cl21))
```

Fix error 'The function CL21.CORE.PACKAGE:IN-PACKAGE is undefined.' by sbcl.
user
```common lisp
(defpackage cl21)
(cl:in-package :cl21)

(cl:dolist (package-name '(:cl21.core))
  (cl:let ((package (cl:find-package package-name)))
    (cl:unless package
      (cl:error "Package \"~A\" doesn't exist." package-name))
    (cl:do-external-symbols (symbol package)
      (cl:shadowing-import symbol)
      (cl:export symbol))))

(cl:in-package :cl-user)
(defpackage cl21-user
  (:use :cl21))
```
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