assert-set statement
====================

Perhaps something like this:

    int x
    if ... {
        ...
        x = 1
    } else if ... {
        ...
        x = 2
    } else if ...
    ...
    } else {
        x = 9
    }
    assert is_set x

And the inverse:

    assert is_unset x

Other useful things:

    assert lifetime x >= y

    assert low_bound(x) <= high_bound(y)