dsl_arithmetic_5_expression.h

using nb = int;
using id = std::string;

struct add_tag {};
struct mul_tag {};

template<typename Tag, typename R>
struct op
{
   op() = default;

   template<typename Range>
   explicit op (Range const& rng) : m_rands(rng.begin(), rng.end()) {}
   
   std::vector<R> const& rands() const { return m_rands; }
   
private:
   std::vector<R> m_rands;
};

template<typename R> using add_op = op<add_tag, R>;
template<typename R> using mul_op = op<mul_tag, R>;

template<typename R>
using expression_r = boost::variant<int, id, add_op<R>, mul_op<R>>;