$ cat foo.scm
(define (foo2a x1 x2 #!optional (x3 -3) (x4 -4))
  (for-each display (list x3 "   " x4 "   " (integer? x3) "   " (integer? x4) #\newline))
  (list x1 x2 x3 x4))

(define (test)
  (equal? '(1  2  -3  -4) (foo2a 1 2)))

(display '(1  2  -3  -4))
(newline)
(display (foo2a 1 2)) ;; --> (1 2 -3 -4)
(newline)
(display (test)) ;; --> #f
(newline)
$ ./foo1
(1 2 4294967293 4294967292)
-3   -4   #t   #t
(1 2 -3 -4)
-3   -4   #t   #t
#f
$ exit