// In header: <boost/accumulators/accumulators_fwd.hpp>
template<typename Accumulator> 
struct droppable_accumulator_base : public Accumulator {
  // construct/copy/destruct
  template<typename Args> droppable_accumulator_base(Args const &);
  droppable_accumulator_base(droppable_accumulator_base 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;
};