def zeroth_order(t: float, q: list, k: float) -> float:
    """Simple zeroth-order rate function.

    :param t:   model time
    :param q:   Vector (list) of mass distribution through compartments (not actually used but required argument for ODE solver)
    :param k:   Time constant of the rate function.
    :returns:   Zeroth-order rate.
    """
    return k