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

It is necessary to summon and cite the defendant.  Even the pope may not omit the summons, and even less another judge may not omit a summons because necessity demands it .  .  . The summons is based on natural law. Consequently the pope cannot bring someone to trial without a summons.