If the Pope Teaches Heresy

In the reading for tomorrow's Catholic Social Teaching class: the 1911 Catholic Encyclopedia's entry on "Infallibility" It's mostly stuff you already knew, but it did contain this curious passage:

A similar exceptional situation might arise were a pope to become a public heretic, i.e., were he publicly and officially to teach some doctrine clearly opposed to what has been defined as de fide catholicâ. But in this case many theologians hoId that no formal sentence of deposition would be required, as, by becoming a public heretic, the pope would ipso facto cease to be pope. This, however, is a hypothetical case which has never actually occurred; even the case of Honorius, were it proved that he taught the Monothelite heresy, would not be a case in point.

Does this strike anyone else as somewhat question-begging? If the pope attempts to infallibly teach something that seems like heresy to you, do you a) assume it's not heresy because the pope is protected by his charism of infallibility* or b) assume he's no longer the pope because he's taught heresy? Does Catholic teaching provide a rule of decision on this issue? Are we all just cafeteria Sedevacantists?

*Which - let's be honest - sounds like something you'd use to battle the Rust Monster in Dungeons & Dragons.