implementation of CL:MAP-INTO

; See http://stackoverflow.com/a/27670789/1281433

(defun %map-into (result-list function list &rest lists)
  "Like CL:MAP-INTO, but only accepts lists (as opposed
to sequences)."
  (apply 'mapl
         (lambda (result-tail &rest args)
           (setf (first result-tail)
                 (apply function (mapcar 'first args))))
         result-list
         list
         lists))

(let ((a '(1 2 3 4))
      (b '(5 6 7 8)))
  (%map-into a '+ a b)
  a)
;=> (6 8 10 12)