...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::accumulators::tag::external
// In header: <boost/accumulators/accumulators_fwd.hpp> template<typename Type, typename Tag = void, typename AccumulatorSet = void> struct external : public boost::accumulators::depends_on< reference< AccumulatorSet, Tag > > { };