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_9 <-> u. p_4 <-> p_9.
p_9 -> u can be inferred via the biconditional elimination rule. p_4 -> p_9 can be inferred via the biconditional elimination rule. u -> p_9 can be inferred via the biconditional elimination rule. p_9 -> p_4 can be inferred via the biconditional elimination rule.