value - an object.

(set symbol value) ==(setf (symbol-value symbol) value)

 (setf (symbol-value 'n) 1)  1
 (set 'n 2)  2
 (symbol-value 'n)  2
 (let ((n 3))
   (declare (special n))
   (setq n (+ n 1))
   (setf (symbol-value 'n) (* n 10))
   (set 'n (+ (symbol-value 'n) n))
   n)  80
 n  2
 (let ((n 3))
   (setq n (+ n 1))
   (setf (symbol-value 'n) (* n 10))
   (set 'n (+ (symbol-value 'n) n))
   n)  4
 n  44
 (defvar *n* 2)
 (let ((*n* 3))
   (setq *n* (+ *n* 1))
   (setf (symbol-value '*n*) (* *n* 10))
   (set '*n* (+ (symbol-value '*n*) *n*))
   *n*)  80
  *n*  2
 (defvar *even-count* 0)  *EVEN-COUNT*
 (defvar *odd-count* 0)  *ODD-COUNT*
 (defun tally-list (list)
   (dolist (element list)
     (set (if (evenp element) '*even-count* '*odd-count*)
          (+ element (if (evenp element) *even-count* *odd-count*)))))
 (tally-list '(1 9 4 3 2 7))   NIL
 *even-count*  6
 *odd-count*  20

set cannot change the value of a lexical variable.