r4 = addMul64(r4, a3, b1) r4 = addMul64(r4, a4, b0) // After the multiplication, we need to reduce (carry) the five coefficients // to obtain a result with limbs that are at most slightly larger than 2⁵¹, // to respect the Element invariant. // // Overall, the reduction works the same as carryPropagate, except with // wider inputs: we take the carry for each coefficient by shifting it right // by 51, and add it to the limb above it. The top carry is multiplied by 19 // according to the reduction identity...