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 Sarah were a musician, then Mary is a janitor. If Charles were fast, then Mary is a janitor. If Mary were a janitor, then Charles is fast. Charles is not fast.
Mary is not a janitor can be inferred via the modus tollens rule. Charles is fast if and only if Mary is a janitor can be inferred via the biconditional introduction rule. Mary is a janitor if and only if Charles is fast can be inferred via the biconditional introduction rule.