You're right for b1 = b2 xor s in the second bullet, I corrected it. You cannot solve b3 before you solve s2. b4 is solved with s6 right after that you realize

You're right for b1 = b2 xor s in the second bullet, I corrected it. You cannot solve b3 before you solve s2. b4 is solved with s6 right after that you realize that s1 = b1, and thus s6 = b1 xor b4