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

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

##</code><code>##

(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)))
##</code><code>##

;;With loop:

(defun pack(lst &optional groups)
  (loop for el in lst 
        for first-group = (when (equal el (caar groups)) (pop groups))
        do (push (cons el first-group) groups))
  (reverse groups))

;;With recursion:

(defun pack(lst &optional groups)
  (if (not lst) (reverse groups) 
      (let* ((el (pop lst))
             (first-group (when (equal el (caar groups)) (pop groups))))
        (pack lst (cons (cons el first-group) groups)))))

;;With no mercy:

(defun pack(lst &optional g)
  (if (not lst) (reverse g) 
      (pack (cdr lst) (cons (cons (car lst) (when (equal (car lst) (caar g)) (pop g))) g)))))