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(defun pick-new (lst el)
  (if (eql (car (car lst)) el)
      (cons (cons el (car lst)) (cdr lst))
...

(defun pack (lst)
  (reverse (reduce #'pick-new lst :initial-value nil)))

(defun pick-new (lst el)
  (cons (cons el (when (equal el (caar lst)) (pop lst))) lst))

(defun pack (lst)
  (reverse (reduce #'pick-new lst :initial-value nil)))

;;With loop:

...
(defun pack(lst &optional g)
  (if (not lst) (reverse g) 
      (pack (cdr lst) (cons (cons (car lst) (when (equal (car lst) (ca
+ar g)) (pop g))) g)))))