 The operation of expansion for belief sets, \(K+p = \Cn(K\cup \{p\})\), was constructed to ensure that the outcome is logically closed. This is not desirable for belief bases, and therefore expansion on belief bases must be different from expansion on belief sets. For any belief base \(A\) and sentence \(p,\) \(A +' p\), the (non-closing) expansion of \(A\) by \(p\), is the set \(A\cup \{p\}\).