def _as_mu_args(
        mu=None,
        omega=None,
        tau=None,
        # _default={},
        **kwargs):
    """
    utility function to convert model arguments to kernel arguments.
    This renames omega and mu, and *deletes* tau.
    """
    kwargs = dict(**kwargs)
    # kwargs.setdefault(**_default)
    if omega is not None:
        kwargs['kappa'] = omega
    if mu is not None:
        kwargs['mu'] = mu
    return kwargs