基于partial_context再来解释context
namespace detail
{
template<typename... Middlewares>
struct partial_context : public pop_back<Middlewares...>::template rebind<partial_context>, public last_element_type<Middlewares...>::type::context
{
using parent_context = typename pop_back<Middlewares...>::template rebind<detail::partial_context>;
template<int N>
using partial = typename std::conditional<N == sizeof...(Middlewares) - 1, partial_context, typename parent_context::template partial<N>
