```common lisp
(cl:in-package #:clump-2-3-tree)

(defclass tree ()
  ((%contents :initform nil :accessor contents)))

(defclass node ()
  ((%parent :initarg :parent :accessor parent)))

(defclass leaf (node)
  ((%contents :initform nil :accessor contents)))
```

Change from :INITFORM to :INITARG in leaf contents slot.
user
```common lisp
(cl:in-package #:clump-2-3-tree)

(defclass tree ()
  ((%contents :initform nil :accessor contents)))

(defclass node ()
  ((%parent :initarg :parent :accessor parent)))

(defclass leaf (node)
  ((%contents :initarg :contents :accessor contents)))
```
assistant