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: p_4 <-> u. p_4 <-> s.
p_4 -> u can be inferred via the biconditional elimination rule. p_4 -> s can be inferred via the biconditional elimination rule. u -> p_4 can be inferred via the biconditional elimination rule. s -> p_4 can be inferred via the biconditional elimination rule.