What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? q_2 <-> p_7. p_7 -> p_18. ~r_3. p_18 <-> r_3.
q_2 -> p_7. p_18 -> r_3. p_7 -> q_2. r_3 -> p_18. ~p_18.