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

(defgeneric rotate-left (tree))

(defgeneric rotate-right (tree))
```

Add method on ROTATE-LEFT specialized to NODE.
user
```common lisp
(cl:in-package #:clump-binary-tree)

(defgeneric rotate-left (tree))

(defgeneric rotate-right (tree))

(defmethod rotate-left ((tree node))
  (when (null (right tree))
    (error 'right-subtree-must-be-non-nil :tree tree))
```
assistant