"Preamble" the SSPX are supposed to sign is about. Presumably something about the authority of the Sovereign Pontiff, something about the reality of Rome, that it is the city that exists with all the sanity and nonsense of now, rather than some mythical perfect "eternal Rome" that some SSPXers try to imagine, how can one describe them - idealised Ultramontanes?
There is something of the perfectii about some of the Society, which can set up personal judgement against the Church. Quite how the Holy Father and Cardinal Levada can frame a document which divides sede vacantists, from Catholics, is beyond my understanding, especially if one adds into the mix, amongst the French at least, a certain Gallican independence of thought. Simply put, will Bishop Williamson be able to sign it?
Defining "Catholic" is tough, with almost formal schism in Austria on the one hand and Bishop Fellay's men on the other, the Catholic Church is indeed a broad one.
But if we bend over backwards to keep Liberals in the Church how far forwards do we bend to get the extremes of the SSPX in and how extreme can they be?