Consider the following premises. ~u. t -> p. t -> u. u -> t. p -> t. Can we infer ~p from them?
 From t -> u, u -> t we can infer t <-> u. From p -> t, t -> p we can infer p <-> t. From t <-> u, ~u we can infer ~t. Finally, from p <-> t, ~t we can infer ~p. Therefore, the answer is yes.