What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: If it is late, then it is snowing. It is not snowing. It is late if and only if William is a taxi driver. Mary is writing a letter.
It is not late can be inferred via the modus tollens rule. It is not snowing and Mary is writing a letter can be inferred via the conjunction rule. Mary is writing a letter and it is not snowing can be inferred via the conjunction rule. If it is late, then William is a taxi driver can be inferred via the biconditional elimination rule. If William were a taxi driver, then it is late can be inferred via the biconditional elimination rule.