Troyes, Bibliothèque municipale 103, fol. 1r

Gratian, Decretum

Johannes Monachus to Extravag. com. 2.3.1 (Rem non novam) v. Non obstantibus aliquibus privilegiis: 

Necesse est ipsum citari et uocari; nec papa hoc potest omittere, et minus alius iudex, quia sic omitteretur cognitio que ad iudicium de necessitate requiritur . . . Et sic patet secundum et tertium simul, scilicet quod citatio est de iure naturali et per consequens quod papa contra aliquem procedere non potest nisi citatione premissa.

It is neccessary that he be summoned and cited; Not even the pope can omit this, and even less another judge, because it would be to omit the full recognition of the case that necessity requires .  .  . And so it is clear that the second and third  together, namely that the summons is based on natural ius and consequently that the pope cannot try anyone without summoning him first.