def __k_chk(k):
    """
    Ensures a kelvin value is not below absolute zero. If it is
    not, we raise a ValueError instead. If it is valid, we return it.
    """
    if k < 0:
        if k % 1 == 0:
            k = int(k)
        raise ValueError(f'{k}K is below absolute zero, and is invalid.')
    return k