Z x = 2.Z^^100;
Z y = 12;
assert(mul(x, Z(12)) == // l-value, r-value
mul(Z(12), x)); // r-value, l-value
assert(mul(x, y) == // l-value, l-value
mul(2.Z^^100, 12.Z)); // r-value, r-value
assert(mul(12.Z, 2.Z^^100) == // r-value, r-value
mul(2.Z^^100, 12.Z)); // r-value, r-value
Get product of x and y (x + y).