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 janitor, then z_1 is making tea. For all z_1, if z_1 makes tea, then z_1 is a janitor. For all z_1, if z_1 makes tea, then z_1 is a janitor.
For all z_1, z_1 is a janitor if and only if z_1 will make tea can be inferred via the universal biconditional introduction rule. For all z_1, z_1 is a janitor if and only if z_1 will make tea can be inferred via the universal biconditional introduction rule. For all z_1, z_1 will make tea if and only if z_1 is a janitor can be inferred via the universal biconditional introduction rule. For all z_1, z_1 will make tea if and only if z_1 is a janitor can be inferred via the universal biconditional introduction rule.