A series of questions asks me about the "atomic formula" (AF) of a set {A, B, C}. "Atomic formula" isn't defined anywhere in the text nor can I find information on what it is online. Is "atomic formula" a standard math term? The only clue the book gives is that AF$_0$ = {A, B, C}. What is AF$_1$ then? Or AF$_2$?
Thanks!