Splitting argpack in half?

Question

How can I split an argument pack in two equal parts?

For example, I would like to do something like this:

template<typename T> T sum(const T& t) { return t; }  template<typename T> T sum(const T& t1, const T& t2) { return t1 + t2; }  template<typename ...T> T sum(T&& ...t) { sum(first_half(t)...) + sum(second_half(t)...); }