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: q_2 -> t. q_2 <-> u_1. u_1 or ~s.
s -> u_1 can be inferred via the rewriting rule. q_2 -> u_1 can be inferred via the biconditional elimination rule. u_1 -> q_2 can be inferred via the biconditional elimination rule.