[#getting_started_37]

  using unary_exp3 = foldr_start_with_parser< minus_token, primary_exp3, boost::mpl::lambda<boost::mpl::negate<boost::mpl::_1>>::type >;
  using mult_exp6 = foldl_start_with_parser< sequence<one_of<times_token, divides_token>, unary_exp3>, unary_exp3, boost::mpl::quote2<binary_op> >;
  struct plus_exp3 : foldl_start_with_parser< sequence<one_of<plus_token, minus_token>, mult_exp6>, mult_exp6, boost::mpl::quote2<binary_op> > {};
  using exp_parser20 = build_parser<plus_exp3>;