//
// q = r s = (u_0 + u_1) (v_0 + v_1)
// = (u_0 v_0) + (u1 v_1) + (u_0 v_1 + u_1 v_0)
- // = a + d + c
+ // = a + c + b
//
// The first two terms we've already calculated; the last is the
// remaining one we want. We'll set B = t^128. We know how to do