Consider the following premises. For all x, x is a sailor or x will not sleep. There is at least one x for which x will sleep if and only if x is a sailor. Can we infer the following from them? If we can, name the inference rule being used: For all x, the fact that x sleeps does not imply that x is a sailor.
 From the fact that there is at least one x for which x will sleep if and only if x is a sailor we can infer that there is at least one x for which if x were a sailor, then x is sleeping via existential biconditional elimination. Finally, from the fact that for all x, x is a sailor or x will not sleep we can infer that for all x, if x sleeps, then x is a sailor via universal rewriting, which contradicts that For all x, the fact that x sleeps does not imply that x is a sailor. Therefore, the answer is no.