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_4, if z_4 were happy, then z_4 is an artist. For all z_4, z_4 is not happy. For all z_4, if z_4 were an artist, then z_4 is happy.
For all z_4, z_4 is not an artist can be inferred via the universal modus tollens rule. For all z_4, z_4 is happy if and only if z_4 is an artist can be inferred via the universal biconditional introduction rule. For all z_4, z_4 is an artist if and only if z_4 is happy can be inferred via the universal biconditional introduction rule.