Coba jelasin Nota dari Paus Gregorius IV dalam bahasa melayu dong

Levillain, Philippe, The Papacy: Gaius-Proxies, Routledge (2002)

Mann, Horace K., The Lives of the Popes in the Early Middle Ages, Vol. II: Paus Semasa Empayar Carolingian, 795-858 (1906)

DeCormenin, Louis Marie; Gihon, James L., A Complete History of the Popes of Rome, from Saint Peter, the First Bishop to Pius the Ninth (1857)