premise: Giovanni Visconti — according to Lorenzo Cardella nephew of Pope Gregory X. He was ostensibly created cardinal-bishop of Sabina by his uncle in 1275 and in 1276 was named judge in the case concerning the translation of bishop Giovanni of Potenza to the archbishopric of Monreale, postulated by the cathedral chapter of Monreale. He died in 1277 or 1278.
hypothesis: Giovanni Visconti died in both 1277 and 1278.
reason: He died in one of the two listed years, not both, as reflected by the word "or" in the text.