What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? For all z_1, if z_1 writes a letter, then z_1 is climbing a mountain. There is at least one z_1 for which z_1 will climb a mountain if and only if z_1 will write a letter. For all z_1, if z_1 climbs a mountain, then z_1 is writing a letter.
For all z_1, z_1 will write a letter if and only if z_1 will climb a mountain. For all z_1, z_1 will climb a mountain if and only if z_1 will write a letter. There is at least one z_1 for which if z_1 climbs a mountain, then z_1 is writing a letter. There is at least one z_1 for which if z_1 writes a letter, then z_1 is climbing a mountain.