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 or ~u_1. u <-> u_1.
u_1 -> u can be inferred via the rewriting rule. u -> u_1 can be inferred via the biconditional elimination rule. u_1 -> u can be inferred via the biconditional elimination rule.