(defun flatten-children-at-depth (tree depth)
  (cond ((minusp depth) tree)
        ((zerop depth)  (if (tree-leaf-p tree)
                            tree
                            (tree-make-tree (tree-label tree)
                                            (mapcan (lambda (child)
                                                      (if (tree-leaf-p child)
                                                          (list child)
                                                          (copy-list (tree-children child))))
                                                    (tree-children tree)))))
        (t             (if (tree-leaf-p tree)
                           tree
                           (tree-make-tree (tree-label tree)
                                           (mapcar (lambda (child)
                                                     (flatten-children-at-depth child (- depth 1)))
                                                   (tree-children tree)))))))

(defun flatten-deepest-children (tree)
  (flatten-children-at-depth tree (- (depth tree) 3)))

(flatten-deepest-children (tree-make-tree nil (list (tree-make-leaf 'a) (tree-make-tree nil (list (tree-make-leaf 'b))) (tree-make-leaf 'c))))
;; -> (a b c)

(flatten-deepest-children
 ;; (a (b (c) d) e ((f g) h i) j)
 (tree-make-tree nil (list (tree-make-leaf 'a)
                           (tree-make-tree nil (list (tree-make-leaf 'b)
                                                     (tree-make-tree nil (list (tree-make-leaf 'c)))
                                                     (tree-make-leaf 'd)))
                           (tree-make-leaf 'e)
                           (tree-make-tree nil (list (tree-make-tree nil (list (tree-make-leaf 'f)
                                                                               (tree-make-leaf 'g)))
                                                     (tree-make-leaf 'h)
                                                     (tree-make-leaf 'i)))
                           (tree-make-leaf 'j))))
;; --> (a (b c d) e (f g h i) j)



(flatten-deepest-children
 ;; (a (b (c) d) e ((f (e a d) g) h i) j)
 (tree-make-tree nil (list (tree-make-leaf 'a)
                           (tree-make-tree nil (list (tree-make-leaf 'b)
                                                     (tree-make-tree nil (list (tree-make-leaf 'c)))
                                                     (tree-make-leaf 'd)))
                           (tree-make-leaf 'e)
                           (tree-make-tree nil (list (tree-make-tree nil (list (tree-make-leaf 'f)
                                                                               (tree-make-tree nil (list (tree-make-leaf 'e)
                                                                                                         (tree-make-leaf 'a)
                                                                                                         (tree-make-leaf 'd)))
                                                                               (tree-make-leaf 'g)))
                                                     (tree-make-leaf 'h)
                                                     (tree-make-leaf 'i)))
                           (tree-make-leaf 'j))))
;; --> (a (b (c) d) e ((f e a d g) h i) j)