Modulus operator
----------------

How should the modulus operator work with negative operands?

    r = a mod b

For `a`, I think it should work as follows:

    a   -5  -4  -3  -2  -1   0   1   2   3   4   5
    b    3   3   3   3   3   3   3   3   3   3   3
    r    1   2   0   1   2   0   1   2   0   1   2

For `b`, I think negative values are counter-intuitive.
For now I will simply disallow negative values.
(It is already required that it must be <> 0, so it actually
only simplifies the code)