#include <cstdio>
#include <tuple>

/* unchanged from et/1_first_draft.cpp */
struct Add {
	template<typename T>
	static auto eval(T lhs, T rhs) {
		return lhs + rhs;
	}
};

struct Mul {
	template<typename T>
	static auto eval(T lhs, T rhs) {
		return lhs * rhs;
	}
};

template<typename T>
class Value {
	T _value;

public:
	Value(T value): _value(value) {}

	auto eval() const { return _value; }
};
/* end of unchanged */

template<typename Op, typename... Operands>
class Expr {
	std::tuple<Operands...> _operands;

public:
	Expr(Operands... operands): _operands{operands...} {}

	auto eval() const {
		return eval(std::make_index_sequence<sizeof...(Operands)>{});
	}

private:
	template<std::size_t... Is>
	auto eval(std::index_sequence<Is...>) const {
		return Op::eval(std::get<Is>(_operands).eval()...);
	}
};

/* unchanged from et/1_first_draft.cpp */
int main() {
	using SubExpr = Expr<Mul, Value<int>, Value<int>>;
	using MyExpr  = Expr<Add, Value<int>, SubExpr>;

	int volatile v0 = 3, v1 = 5, v2 = 7;
	int result;

	MyExpr expr{{v0}, {{v1}, {v2}}};
	result = expr.eval();

	std::printf("%d\n", result);
}
/* end of unchanged */