c1+c4,c2+c3
a+b 0 0 b
0 a+b b 0
0 a+b a 0
a+b 0 0 a
r4-r1,r3-r2
0 0 a-b 0
0 0 0 a-b
= (a+b)^2 (a-b)^2.