concept Point2DConcept<typename T> : PointNDConcept<T> {
where num_dimensions == 2;
where SameType<axis<0>::type, axis<1>::type>;
typename value_type = axis<0>::type;
const value_type& operator[](const T&, size_t i);
value_type& operator[]( T&, size_t i);
value_type x,y;
};