...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<Feature, Tag, void>
template<typename Feature, typename Tag> struct external<Feature, Tag, void> : public boost::accumulators::depends_on< Feature1, Feature2, > { // types typedef unspecified impl; };