  Nothing prevents both \(A\) and \(\twiddle A\) being true at some worlds; indeed, respecting relevance demands it. These conditions can be seen as generalizations of more standard ones: in the case of implication, if \(y = z\) then this is just a modalized material conditional in a Kripke frame; in the case of negation, if \(x = x^*\) then this is just the clause for negation in classical logic. For further details, see the entry on relevance logic, Mares (2004), and Priest (2008, ch. 10). On the Routley star see Restall (1999).