So what did the Emperor Charlemagne do, given the impossible situation he was landed in by Pope Hadrian's lack of interest in Liturgical Uniformity? And the by fact that the book the pope had eventually sent him seemed unusable?
He got his scholars to provide an Appendix to Pope Hadrian's book. You see, that book didn't even contain liturgical provision for what we would call the 'Green Sundays' of the year! Or the Exsultet!
And this Appendix was mightily long! In the HBS edition, the basic papal part of the book runs to 143 pages; but the Appendix is considerably longer, with 177 pages! The two parts are divided by a Praefatiuncula usually referred to (from its first word) as the Praefatiuncula hucusque. In this, the Editors explain that 'we' velut flores pratorum vernantes had plucked, brought together, corrected ... etc.. the material in the Appendix from other sources.
They go on to make clear that they expect there to be two classes of user: those (si cui placent ... suscipere) who will use the Appendix; and (Si vero superflua vel non necessaria sibi illa iudicaverit) those who will use only the first section and will ignore the Appendix.
If this doesn't mean that there were to be two forms of the Rite, I don't know what would!
The two categories of users were expected to behave "placabiliter"1
Ever since Summorum Pontificum, there have been pompous know-alls who have paraded around ridiculing the canonical determination of Benedict XVI that there could be two 'Forms' of the Roman Rite.
In fact, there always have been plural forms of the Roman Rite ... Archbishop Cranmer listed what there was in England alone: "some folowyng Salisbury use, some Herford use, some the use of Bangor, some of Yorke, and some of Lincolne". When I was an undergraduate, one could go to Blackfriars for the Dominican Rite, or to the (then nearly extinct) Jesuits of Alyoggers for the edition of S Pius V.
It was Pope Francis himself who actually promulgated the Ordinariate Missal as being a lawful use of the Roman Rite.
The present, 'momentous' 'discovery' of PF that there can only be an unicus usus of the Roman Rite is as unhistorical as it is unpastoral.
It is ridiculous. It is ultra vires. Barmy.