Объясните один момент, касающийся переменных

(defun compress (w &optional b)
  (cond ((null w) nil)
        ((equal (car w) b) (compress (cdr w) b))
        ((cons (car w) (compress (cdr w) (car w))))))

;;;; файл
(defun f1 (&optional x)
  (format t "x: ~S~%" x))
 
(defun f2 (&optional (x 1))
  (format t "x: ~S~%" x))
 
(defun f3 (&optional (x nil x-supplied-p))
  (unless x-supplied-p
    (write-line "Using the default value of x."))
  (format t "x: ~S~%" x))
 
;;;; REPL
 
TEST> (f1)
x: NIL
NIL
TEST> (f1 1)
x: 1
NIL
TEST> (f2)
x: 1
NIL
TEST> (f1 3)
x: 3
NIL
TEST> (f3)
Using the default value of x.
x: NIL
NIL
TEST> (f3 3)
x: 3
NIL
TEST> (f3 nil)
x: NIL
NIL

(defun over-compress (w)
  (compress w nil))
 
(defun compress (w b)
  (cond ((null w) nil)
        ((equal (car w) b) (compress (cdr w) b))
        ((cons (car w) (compress (cdr w) (car w))))))
 
> (over-compress '(a a b c c c d e e))
(A B C D E)

(defun compress (w)
  (cond ((null (cdr w)) w)
        ((equal (car w) (cadr w)) (compress (cdr w)))
        ((cons (car w) (compress (cdr w))))))
 
> (compress '(a a))
(A)
> (compress '(a))
(A)
> (compress '())
NIL
> (compress '(a b))
(A B)
> (compress '(a a b c c c d e e))
(A B C D E)