Consider the following premises. p_13 <-> p_18. p_18 <-> w. p_13 or ~w. Can we infer p_18 <-> p_13 from them?
 From p_18 <-> w we can infer p_18 -> w. From p_13 <-> p_18 we can infer p_13 -> p_18. From p_13 or ~w we can infer w -> p_13. From p_18 -> w, w -> p_13 we can infer p_18 -> p_13. Finally, from p_18 -> p_13, p_13 -> p_18 we can infer p_18 <-> p_13. Therefore, the answer is yes.