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: t -> p_1. p_1 -> t. p_1 <-> u. ~t.
~p_1 can be inferred via the modus tollens rule. t <-> p_1 can be inferred via the biconditional introduction rule. p_1 <-> t can be inferred via the biconditional introduction rule. p_1 -> u can be inferred via the biconditional elimination rule. u -> p_1 can be inferred via the biconditional elimination rule.