 The proof is based on turning the two disjunctive premises into the conditional statements that logically follow from them, namely \((\aD^{3})r\neg q \vdash (\aC)\neg r\neg q\), and \((\aD^{1})pr \vdash (\aC)p\neg r\). From such conditional premises, one may easily derive the conditional conclusion \((\aC)p\neg q\), which in turn entails the disjunctive proposition \((\aD^{1/3})\neg p\neg q\).