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Coding Standards
This is an older version of Boost and was released in 2022. The current version is 1.90.0.
boost::accumulators::tag::as_droppable<droppable< Feature >>
// In header: <boost/accumulators/framework/accumulators/droppable_accumulator.hpp> template<typename Feature> struct as_droppable<droppable< Feature >> { // types typedef droppable< Feature > type; };