def get_omega_k_0(**cosmo):
    """'Spatial curvature density' omega_k_0 for a cosmology (if needed).

    If omega_k_0 is specified, return it. Otherwise return:

      1.0 - omega_M_0 - omega_lambda_0

    """
 
    if 'omega_k_0' in cosmo:
        omega_k_0 = cosmo['omega_k_0']
    else:
        omega_k_0 = 1. - cosmo['omega_M_0'] - cosmo['omega_lambda_0']
    return omega_k_0