 Belief revision theory represents beliefs with sentences of propositional logic: \(A\), \(A \supset B\), \(\neg (A \wedge \neg B)\), and so on. Your full corpus of beliefs is a set of such sentences we call \(K\) (not to be confused with the sentential operator \(K\) from epistemic logic  (§4.1)).  Importantly, we assume that \(K\) contains everything entailed by your beliefs: if \(A\) and \(A \supset B\) are in \(K\), then so is \(B\), for example.