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