template< size_t order, typename Iterator>
std::array<typename std::iterator_traits<Iterator>::value_type, order >
polynomial_fit(Iterator &xv_begin, Iterator &xv_end, Iterator &yv_begin, Iterator &yv_end)
{
	//some logic
	std::array<typename std::iterator_traits<Iterator>::value_type, order > ret;
	//deep copy into ret (order < 5)
	return ret;
}