What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? r_4 or q. p_2 -> ~u_3. r_4 -> u_3. q -> p_20. p_20 -> u_3. u_3 -> p_20.
r_4 -> p_20. q -> u_3. p_20 <-> u_3. u_3 <-> p_20.