namespace boost {
  namespace yap {
    template<expr_kind Kind> struct expr_tag;
    template<typename Expr> struct is_expr;
    template<expr_kind Kind, typename Tuple> struct minimal_expr;
    template<long long I> struct placeholder;

    enum expr_kind;
    typedef expr_template< expr_kind::terminal, hana::tuple< T > > terminal;
    typedef expr_template< expr_kind::expr_ref, hana::tuple< std::remove_reference_t< T > * > > expression_ref;
    namespace literals {
      template<char... c> constexpr auto operator""_p();
    }
  }
}