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: Mary is not poor. If it is late, then Mary is poor. If Mary were poor, then it is late. If it is late, then Mary is poor.
It is not late can be inferred via the modus tollens rule. It is not late can be inferred via the modus tollens rule. It is late if and only if Mary is poor can be inferred via the biconditional introduction rule. Mary is poor if and only if it is late can be inferred via the biconditional introduction rule. Mary is poor if and only if it is late can be inferred via the biconditional introduction rule. It is late if and only if Mary is poor can be inferred via the biconditional introduction rule.