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 <-> q_4. q_1 -> q_4. q_4 -> q_1.
q_1 <-> q_4 can be inferred via the biconditional introduction rule. q_4 <-> q_1 can be inferred via the biconditional introduction rule. u -> q_4 can be inferred via the biconditional elimination rule. q_4 -> u can be inferred via the biconditional elimination rule.