def add_extras(cosmo):
    """Sets neutrino number N_nu = 0, neutrino density
       omega_n_0 = 0.0, Helium mass fraction Y_He = 0.24.
       Also sets w = -1.
    """
    extras = {'omega_n_0': 0.0, 'N_nu': 0, 'Y_He': 0.24, 'w': -1.0,
        'baryonic_effects': False}
    cosmo.update(extras)
    return cosmo
