Define a recursive lambda.

dump-tree.lisp

; dump tree with a recursive lambda
(funcall
 (fn dump-tree (e)
     (if (atom e)
         (format t "~A~%" e)
       (mapc #'dump-tree e)))
 '(1 (2 (3 (4) 5) 6) 7 (8 9)))

fn.lisp

(defmacro fn (&rest body)
  "(fn name? [param*] body)

  Define a recursive lambda."
  (if (listp (car body))
    `(lambda ,@body)
    `(lambda (&rest args)
       (labels (,body)
         (apply (function ,(car body)) args)))))