...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::numeric::functional::outer_product_base
// In header: <boost/accumulators/statistics/covariance.hpp> template<typename Left, typename Right, typename EnableIf = void> struct outer_product_base { };