Consider the following premises. t -> q_16. q_20. r_2 -> ~q_20. r_2 <-> q_16. Can we infer ~t or s_4 from them? If possible, name the inference rules being used at each step.
 From r_2 -> ~q_20, q_20 we can infer ~r_2 via modus tollens. From r_2 <-> q_16, ~r_2 we can infer ~q_16 via biconditional elimination. From t -> q_16, ~q_16 we can infer ~t via modus tollens. Finally, from ~t we can infer ~t or s_4 via addition. Therefore, the answer is yes.