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_6. ~q_4. u <-> q_4. q_6 -> u.
u <-> q_6 can be inferred via the biconditional introduction rule. q_6 <-> u 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. ~u can be inferred via the biconditional elimination rule.