Use of a counter in a list

Hi,

I don't understand why this code does not work with SBCL while it works perfectly with ABCL and CLISP

(let ((a '(0)))
(defun inc-a () (incf (car a))) (defun a () (car a)))

On SBCL

CL-USER> (a)
0 (0 bits, #x0, #o0, #b0)
CL-USER> (inc-a)
1 (1 bit, #x1, #o1, #b1)
CL-USER> (a)
0 (0 bits, #x0, #o0, #b0)CL-USER>

On CLISP or ABCL

[2]> (a)
0
[3]> (inc-a)
1
[4]> (a)
1[5]>

Even more worrying to me, I get the same result with (sb-ext:atomic-incf), the (a) function always returns 0.

Please, can you explain to me like I'm 5 years old why it does not work on SBCL and why it works with other LISP compilers ?

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