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: r_1. p_20 <-> t. p_20 -> ~r_1. p_4 -> t.
~p_20 can be inferred via the modus tollens rule. p_20 -> t can be inferred via the biconditional elimination rule. t -> p_20 can be inferred via the biconditional elimination rule.