Consider the following premises. Patricia is writing a letter if and only if Robert is a sailor. Patricia is not writing a letter. For all x_18, x_18 is an artist. Can we infer the following from them? If we can, name the inference rule being used: Robert is a sailor.
 From the fact that for all x_18, x_18 is an artist we can infer that Susan is an artist via universal instantiation. Finally, from the fact that Patricia is writing a letter if and only if Robert is a sailor, and that Patricia is not writing a letter we can infer that Robert is not a sailor via biconditional elimination, which contradicts that Robert is a sailor. Therefore, the answer is no.