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 overcast, then Sarah is a taxi driver. For all z, z will write a letter if and only if z is a taxi driver. Sarah is not writing a letter.
For all z, if z writes a letter, then z is a taxi driver can be inferred via the universal biconditional elimination rule. For all z, if z were a taxi driver, then z is writing a letter can be inferred via the universal biconditional elimination rule.