l0×l2 + l1×l1 + l2×l0 + 19×(l3×l4 + l4×l3) = 2×l0×l2 + l1×l1 + 19×2×l3×l4 r2 := mul64(l0_2, l2) r2 = addMul64(r2, l1, l1) r2 = addMul64(r2, l3_38, l4) // r3 = l0×l3 + l1×l2 + l2×l1 + l3×l0 + 19×l4×l4 = 2×l0×l3 + 2×l1×l2 + 19×l4×l4 r3 := mul64(l0_2, l3) r3 = addMul64(r3, l1_2, l2) r3 = addMul64(r3, l4_19, l4) // r4 = l0×l4 + l1×l3 + l2×l2 + l3×l1 + l4×l0 = 2×l0×l4 + 2×l1×l3 + l2×l2 r4 := mul64(l0_2, l4) r4 = addMul64(r4, l1_2, l3) r4 = addMul64(r4, l2, l2) c0 := shiftRightBy51(r0) c1 := shiftRightBy51(r1)...