Consider the following premises. p_18 <-> u_2. u_2 <-> q_13. q_13. Can we infer p_18 from them?
 From p_18 <-> u_2 we can infer u_2 -> p_18. From u_2 <-> q_13 we can infer q_13 -> u_2. From q_13 -> u_2, q_13 we can infer u_2. Finally, from u_2 -> p_18, u_2 we can infer p_18. Therefore, the answer is yes.