// F:
// template<size_t index, class... Args>
// Ret operator()(std::integral_constant<size_t, index>, Args&&... args);
//
// we want to call F(Args...) with index value given at runtime


namespace detail
{
   template<size_t Head, size_t... Tail>
   size_t seq_head = Head;


   template <class F, class... Args, size_t... Idxs>
   auto unroll_conseq_impl(std::index_sequence<Idxs...>, size_t index, F&& f, Args&&... args)
       -> decltype(std::declval()(std::integral_constant<size_t, seq_head<Idxs...>>{}, std::forward(args)...))
   {
       using Ret = decltype(std::declval()(std::integral_constant<size_t, seq_head<Idxs...>>{}, std::forward(args)...));
       typedef Ret(*Callback)(Args...);
       constexpr size_t length = sizeof...(Idxs);
       constexpr std::array<Callback, length> callbacks = {
           [](F& cf, Args&&... cargs) { return cf(std::integral_constant<size_t, Idxs>{}, std::forward(cargs)...); }...
       };

       return callbacks[index](f, std::forward(args)...);
   }
}
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