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 <-> s. s -> q_1. q_1 -> s. ~q_1.
~s can be inferred via the modus tollens rule. s <-> q_1 can be inferred via the biconditional introduction rule. q_1 <-> s can be inferred via the biconditional introduction rule. p -> s can be inferred via the biconditional elimination rule. s -> p can be inferred via the biconditional elimination rule.