template<typename P>
struct boost::gil::PointNDConcept< P >
N-dimensional point concept. 
concept PointNDConcept<typename T> : Regular<T>
{
    
    template <size_t K>
    struct axis; where Metafunction<axis>;
    const size_t num_dimensions;
    
    template <size_t K>
    typename axis<K>::type const& T::axis_value() const;
    template <size_t K>
    typename axis<K>::type& T::axis_value();
};