...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
template <int I> static inline scalar_type & write_element( Vector & v );
This function template is expected to be static member of user-defined specializations of the vec_traits template. It returns mutable reference to the I-th element of v.