What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? p_15 -> q_15. q_15 -> ~s_4. s_4. ~p_15 -> u_1. q_15 -> p_15.
~q_15. p_15 -> ~s_4. p_15 <-> q_15. q_15 <-> p_15.