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: u_2 <-> p_11. p_11 -> ~t_4. t_4. s -> u_2.
~p_11 can be inferred via the modus tollens rule. u_2 -> p_11 can be inferred via the biconditional elimination rule. p_11 -> u_2 can be inferred via the biconditional elimination rule.