how can i reimplement backquote in common lisp?

I have to remake backquote (with unquote and unquote-splicing) without using the builtins reader macros `,@ The behaviour expected is:

> (BACKQUOTE (A B (LIST ‘C ‘D) (COMA (LIST ‘E ‘F)
                               (COMA-AT (LIST ‘G ‘H)开发者_高级运维))
(A B (LIST ‘C ‘D) (E F) G H)

I try to do it with a macro but the results are no the expected.

Many thanks!!!

Any hints of what could be done?

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http://lib.store.yahoo.net/lib/paulgraham/glsbq.lisp has an example

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For Those Who well settle for a simple and incorrect solution but it works, while trying to understand the paul graham code:

(defmacro backquote (expr)
  (labels
      ((step (p n)
             (append p
                     (if (atom n) (list n)
                       (case (car n)
                         ('comma (list (eval (cadr n))))
                         ('comma-at (eval (cadr n))))))))
    (list 'quote (reduce #'step (cons () expr)))))

corrections and suggestions to improve it are welcome!