let mut sum = 0;
for (blk1, blk2) in mb1.coeffs.iter().zip(mb2.coeffs.iter()) {
for (&c1, &c2) in blk1.iter().zip(blk2.iter()) {
let mut sum = 0;
for (blk1, blk2) in mb1.coeffs.iter().zip(mb2.coeffs.iter()) {
for (&c1, &c2) in blk1.iter().zip(blk2.iter()) {