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_4 <-> s_1. s_1 or ~u_2.
u_2 -> s_1 can be inferred via the rewriting rule. p_4 -> s_1 can be inferred via the biconditional elimination rule. s_1 -> p_4 can be inferred via the biconditional elimination rule.