Consider the following premises. p_17 <-> u_2. q. u_2 <-> u. ~p_17. u <-> r. Can we infer r from them? If possible, name the inference rules being used at each step.
 From p_17 <-> u_2, ~p_17 we can infer ~u_2 via biconditional elimination. From u_2 <-> u, ~u_2 we can infer ~u via biconditional elimination. Finally, from u <-> r, ~u we can infer ~r via biconditional elimination, which contradicts r. Therefore, the answer is no.