reduce_once
return reduce_once(remainder);
s->c[j] = reduce_once(odd + even);
s->c[j + offset] = reduce_once(even - odd +
s->c[j] = reduce_once(odd + even);
lhs->c[i] = reduce_once(lhs->c[i] + rhs->c[i]);
lhs->c[i] = reduce_once(lhs->c[i] - rhs->c[i] + kPrime);