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_1, if z_1 were a cashier, then z_1 is climbing a mountain. For all z_1, z_1 will listen to a song if and only if z_1 is a cashier. There is at least one z_1 for which if z_1 climbs a mountain, then z_1 is a cashier.
There is at least one z_1 for which z_1 will climb a mountain if and only if z_1 is a cashier can be inferred via the existential biconditional introduction rule. There is at least one z_1 for which z_1 is a cashier if and only if z_1 will climb a mountain can be inferred via the existential biconditional introduction rule. For all z_1, if z_1 listens to a song, then z_1 is a cashier can be inferred via the universal biconditional elimination rule. For all z_1, if z_1 were a cashier, then z_1 is listening to a song can be inferred via the universal biconditional elimination rule.