(labels ((mapatoms (function tree)
           (cond
             ((atom tree)
              (funcall function tree))
             ((consp tree)
              (cons (mapatoms function (car tree))
                    (mapatoms function (cdr tree)))))))
  (mapatoms (lambda (atom)
              (format t "~A~%" atom)
              (read-from-string (format nil "~S" atom)))
            '(defmacro (x) `(list ,x))))

;; output:
;; DEFMACRO
;; X
;; NIL
;; QUASIQUOTE
;; LIST
;; ,X
;;
;; Comma not inside a backquote.
;;
;;   Stream: #<dynamic-extent STRING-INPUT-STREAM (unavailable) from ",X">
;;    [Condition of type SB-INT:SIMPLE-READER-ERROR]
;;
;; Restarts:
;;  0: [RETRY] Retry SLIME REPL evaluation request.
;;  1: [*ABORT] Return to SLIME's top level.
;;  2: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {1002B59D33}>)