[#getting_started_17]

  template <class L, char Op, class R> struct eval_binary_op;
  template <class L, class R> struct eval_binary_op<L, '+', R> : boost::mpl::plus<L, R>::type {};
  template <class L, class R> struct eval_binary_op<L, '-', R> : boost::mpl::minus<L, R>::type {};