Desugaring &key arguments in CL

(defun foo (&rest arguments)
  ;; this should all be gensymmed
  (let (a-value a-p b-value b-p allow-other-keys allow-other-keys-p other-key-p)
    (loop while arguments do
      (let ((key (pop arguments))
            (value (if (null arguments)
                       (error "Odd number of keyword arguments")
                       (pop arguments))))
        (case key
          (:a
           (unless a-p
             (setf a-value value
                   a-p t)))
          (:b
           (unless b-p
             (setf b-value value
                   b-p t)))
          (:allow-other-keys
           (unless allow-other-keys-p
             (setf allow-other-keys value
                   allow-other-keys-p t)))
          (t
           (setf other-key-p t)))))
    (when (and other-key-p
               (not allow-other-keys))
      (error "Unknown keyword argument(s)"))
    (let* ((a (if a-p
                  a-value
                  (a-default-form)))
           (b (if b-p b-value b-default-form)))
      ...)))