From the "Master of Sentences," he had passed to the "Capitularies of Charlemagne;" and he had devoured in succession, in his appetite for science, decretals upon decretals, those of Theodore, Bishop of Hispalus; those of Bouchard, Bishop of Worms; those of Yves, Bishop of Chartres; next the decretal
of Gratian, which succeeded the capitularies of Charlemagne; then the collection of Gregory IX.
Finally there was Raymond of Penyafort, who compiled the Decretals
of canon law of Hugolino-Gregory IX (pope from 1227-41), helping Dominicans deal with both Gregory's work and Gratian's Decretals, both important to their ministry.
Christopher de Hamel identified the parchment layers' first use as a Bolognese manuscript of the decretals
of Gregory IX from approximately 1300, with a gloss, and the paper layer is coarse and blank.
See also Hartmann & Pennington: The History of Medieval Canon Law in the Classical Period, 1140-1234: From Gratian to the Decretals
of Pope Gregory LX, (CUA Press, 2008),p.
The Authoritative Text: Raymond of Penyafort's editing of the Decretals
of Gregory IX (1234), Columbia University, 2011.
If the Code of Canon Law, in distinction from the Decretals
of Gratian, does not explicitly address the issue of heretical Popes, one cannot take this silence to mean the rejection either of the possibility of a heretical Pope or of the possibility that a judgment statement could be made.
535, "the second necessary condition for the tax being fair is the final reason or cause" as it says in the Decretals
She argues that the text for these romanz Psalms was drawn from another (third) independent translation of the Gallican Psalter that preexisted the Decretals
35) Aquinas is, in fact, quoting the Decretals
of Pope Gregory IX.