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: For all z, if z were an office clerk, then z is a scientist. There is at least one z for which if z were a scientist, then z is an office clerk. For all z, if z were a scientist, then z is not writing a letter. For all z, z will write a letter.
For all z, z is not a scientist can be inferred via the universal modus tollens rule. There is at least one z for which z is a scientist if and only if z is an office clerk can be inferred via the existential biconditional introduction rule. There is at least one z for which z is an office clerk if and only if z is a scientist can be inferred via the existential biconditional introduction rule.