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

template<typename Expr, typename State, typename Data> 
struct impl :  proto::transform_impl< Expr, State, Data > {
  // types
  typedef proto::call<R(A..., ...)>                                  call_;        // For exposition only
  typedef proto::make<R(A..., ...)>                                  make_;        // For exposition only
  typedef typename mpl::if_<proto::is_callable<R>,call_,make_>::type which;        // For exposition only
  typedef typename boost::result_of<which(Expr, State, Data)>::type  result_type;
  // public member functions
  result_type operator()(typename impl::expr_param, 
                         typename impl::state_param, 
                         typename impl::data_param) const;
};