Maestro n° 103 : Page 83 - Gr 80
r4c1 -7- r2c1 =7=3= r2c1 -3- r2c6 =3=9= r2c6 -9- r9c6 =9=4= r9c6 -4- r9c1 =4= r4c1 => r4c1 ≠ 7