...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::units::is_quantity_of_system<quantity< Unit, Y >, System>
// In header: <boost/units/is_quantity_of_system.hpp> template<typename Unit, typename Y, typename System> struct is_quantity_of_system<quantity< Unit, Y >, System> : public boost::units::is_unit_of_system< Unit, System > { };