// In header: <boost/proto/transform/fold.hpp>

template<typename Expr, typename State, typename Data> 
struct impl :  proto::transform_impl< Expr, State, Data > {
  // types
  typedef when<_, Sequence>                                        X;            // For exposition only
  typedef when<_, State0>                                          Y;            // For exposition only
  typedef typename boost::result_of<X(Expr, State, Data)>::type    seq;          // A Fusion sequence, for exposition only
  typedef typename boost::result_of<Y(Expr, State, Data)>::type    state0;       // An initial state for the fold, for exposition only
  typedef unspecified                                              fun;          // fun(d)(s,e) == when<_,Fun>()(e,s,d)
  typedef typename fusion::result_of::fold<seq, state0, fun>::type result_type;
  // public member functions
  result_type operator()(typename impl::expr_param, 
                         typename impl::state_param, 
                         typename impl::data_param) const;
};