Consider the following premises. There is at least one x for which x is an electrician or x is not fast. For all x, x is fast or x is not an electrician. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x for which x is an electrician if and only if x is fast.
 From the fact that there is at least one x for which x is an electrician or x is not fast we can infer that there is at least one x for which if x were fast, then x is an electrician via existential rewriting. From the fact that for all x, x is fast or x is not an electrician we can infer that for all x, if x were an electrician, then x is fast via universal rewriting. Finally, from the fact that for all x, if x were an electrician, then x is fast, and that there is at least one x for which if x were fast, then x is an electrician we can infer that there is at least one x for which x is an electrician if and only if x is fast via existential biconditional introduction. Therefore, the answer is yes.