(a[0][0] + a[0][1]) * (a[1][0] + a[1][1]) =
 C[0] + C[1] + C[2] + C[3],
where
C[0] = a[0][0] * a[1][0]
C[1] = a[0][0] * a[1][1]
C[2] = a[0][1] * a[1][0]
C[3] = a[0][1] * a[1][1]