the columns into wide, overlapping limbs. // r0 = a0×b0 + 19×(a1×b4 + a2×b3 + a3×b2 + a4×b1) r0 := mul(a0, b0) r0 = addMul19(r0, a1, b4) r0 = addMul19(r0, a2, b3) r0 = addMul19(r0, a3, b2) r0 = addMul19(r0, a4, b1) // r1 = a0×b1 + a1×b0 + 19×(a2×b4 + a3×b3 + a4×b2) r1 := mul(a0, b1) r1 = addMul(r1, a1, b0) r1 = addMul19(r1, a2, b4) r1 = addMul19(r1, a3, b3) r1 = addMul19(r1, a4, b2) // r2 = a0×b2 + a1×b1 + a2×b0 + 19×(a3×b4 + a4×b3) r2 := mul(a0, b2) r2 = addMul(r2, a1, b1) r2 = addMul(r2, a2, b0)...

* 19 a2_19 := a2 * 19 a3_19 := a3 * 19 a4_19 := a4 * 19 // r0 = a0×b0 + 19×(a1×b4 + a2×b3 + a3×b2 + a4×b1) r0 := mul64(a0, b0) r0 = addMul64(r0, a1_19, b4) r0 = addMul64(r0, a2_19, b3) r0 = addMul64(r0, a3_19, b2) r0 = addMul64(r0, a4_19, b1) // r1 = a0×b1 + a1×b0 + 19×(a2×b4 + a3×b3 + a4×b2) r1 := mul64(a0, b1) r1 = addMul64(r1, a1, b0) r1 = addMul64(r1, a2_19, b4) r1 = addMul64(r1, a3_19, b3) r1 = addMul64(r1, a4_19, b2) // r2 = a0×b2 + a1×b1 + a2×b0 + 19×(a3×b4 + a4×b3) r2 := mul64(a0, b2) r2 =...