Consider the following premises. s. p_11 -> s. p_11 <-> s. Can we infer p_11 from them? If possible, name the inference rules being used at each step.
 From p_11 <-> s we can infer s -> p_11 via biconditional elimination. From s -> p_11, p_11 -> s we can infer s <-> p_11 via biconditional introduction. From s <-> p_11 we can infer s -> p_11 via biconditional elimination. Finally, from s -> p_11, s we can infer p_11 via modus ponens. Therefore, the answer is yes.