(* Q4, scope rules*)

let fun a() =
        let fun b() = a()+b();      (* ...e1...; *)
            fun c() = a()+b()+c();  (* ...e2...  *)
        in a()+b()+c()              (* ...e3...  *)
        end;

    fun d() =
        let fun e() = a()+d()+e()   (* ...e4...  *)
        in a()+d()+e()              (* ...e5...  *)
        end

in a()+d()                          (* ...e6...  *)
end

(* No other function calls can be added without error. (However it infinite-loops.) *)