 which allows one to show that if \(p \fishhook q\) is a theorem, then so is \({\sim}\Diamond{\sim}p \fishhook{\sim}\Diamond{\sim}q\), i.e., \(\Box p \fishhook \Box q\), expressing the strict implication of the necessity of \(q\) by the necessity of \(p\). S3 adds to S1 the postulate