Consider the following premises. For all x_17, if x_17 were a janitor, then x_17 is a taxi driver. If Richard were fast, then Mary is a janitor. If Mary were a taxi driver, then Richard is fast. Mary is a taxi driver. Can we infer the following from them? If we can, name the inference rule being used: Richard is not fast.
 From the fact that for all x_17, if x_17 were a janitor, then x_17 is a taxi driver we can infer that if Mary were a janitor, then Mary is a taxi driver via universal instantiation. From the fact that if Richard were fast, then Mary is a janitor, and that if Mary were a janitor, then Mary is a taxi driver we can infer that if Richard were fast, then Mary is a taxi driver via transitivity. From the fact that if Mary were a taxi driver, then Richard is fast, and that if Richard were fast, then Mary is a taxi driver we can infer that Mary is a taxi driver if and only if Richard is fast via biconditional introduction. From the fact that Mary is a taxi driver if and only if Richard is fast we can infer that if Mary were a taxi driver, then Richard is fast via biconditional elimination. Finally, from the fact that if Mary were a taxi driver, then Richard is fast, and that Mary is a taxi driver we can infer that Richard is fast via modus ponens, which contradicts that Richard is not fast. Therefore, the answer is no.