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. r_4 -> u. q <-> r. r_4 <-> r.
~r_4 can be inferred via the modus tollens rule. q -> r can be inferred via the biconditional elimination rule. r_4 -> r can be inferred via the biconditional elimination rule. r -> q can be inferred via the biconditional elimination rule. r -> r_4 can be inferred via the biconditional elimination rule.