def unit_fn_dose(t, X):
    """Unit step function (Heaviside) for dosing regimen, with amount X of
        dose administered at every time step.

    :param t: current time point
    :type t: float
    :param X: amount of dose administered
    :type X: float
    :return: dose X administered at time t
    :rtype: float
    """
    return X