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 -> p_20. u_3 or u. p_1. p_1 -> u_1. p_20 or ~u_3.
u_1 can be inferred via the modus ponens rule. u_3 -> p_20 can be inferred via the rewriting rule.