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: s_1 <-> p_4. p_4 <-> u_2. u_2 -> ~q. q.
~u_2 can be inferred via the modus tollens rule. s_1 -> p_4 can be inferred via the biconditional elimination rule. p_4 -> u_2 can be inferred via the biconditional elimination rule. p_4 -> s_1 can be inferred via the biconditional elimination rule. u_2 -> p_4 can be inferred via the biconditional elimination rule.