// In header: <boost/accumulators/framework/accumulators/droppable_accumulator.hpp>

template<typename Accumulator> 
struct droppable_accumulator_base {
  // types
  typedef droppable_accumulator_base base;        
  typedef mpl::true_                 is_droppable;
  typedef Accumulator::result_type   result_type; 

  // construct/copy/destruct
  template<typename Args> droppable_accumulator_base(Args const &);

  // public member functions
  template<typename Args> void operator()(Args const &);
  template<typename Args> void add_ref(Args const &);
  template<typename Args> void drop(Args const &);
  bool is_dropped() const;
};