Consider the following premises. There is at least one x_7 for which x_7 is not an electrician. For all x_7, if x_7 works, then x_7 is a sailor. For all x_7, x_7 is an electrician if and only if x_7 is a sailor. For all x_7, if x_7 were a sailor, then x_7 is working. Can we infer the following from them? If we can, name the inference rule being used: For all x_7, x_7 will work.
 From the fact that for all x_7, x_7 is an electrician if and only if x_7 is a sailor, and that there is at least one x_7 for which x_7 is not an electrician we can infer that there is at least one x_7 for which x_7 is not a sailor via existential biconditional elimination. From the fact that for all x_7, if x_7 works, then x_7 is a sailor, and that for all x_7, if x_7 were a sailor, then x_7 is working we can infer that for all x_7, x_7 will work if and only if x_7 is a sailor via universal biconditional introduction. Finally, from the fact that for all x_7, x_7 will work if and only if x_7 is a sailor, and that there is at least one x_7 for which x_7 is not a sailor we can infer that there is at least one x_7 for which x_7 will not work via existential biconditional elimination, which contradicts that For all x_7, x_7 will work. Therefore, the answer is no.