Collect items with the same head.

gistfile1.lisp

(defun hashtable-to-list (table &aux result)
  (maphash (lambda (key value)
             (push (cons key value) result))
           table)
  result)

(defun collect-into-table (list &key key-fn value-fn &aux (table (make-hash-table)))
  (mapc (lambda (item &aux (key (funcall key-fn item)))
          (setf (gethash key table)
                (nconc (gethash key table)
                       (funcall value-fn item))))
        list)
  table)

(defun example (list)
  (mapcar #'rest
          (sort (hashtable-to-list
                 (collect-into-table list
                                     :key-fn #'first
                                     :value-fn #'rest))
                #'<
                :key #'first)))