boost::yap::argument
// In header: <boost/yap/algorithm.hpp> template<long long I, typename Expr> constexpr decltype(auto) argument(Expr && expr, hana::llong< I > i);
Returns the i-th argument expression in a call expression.
Equivalent to get(expr, i + 1).
get(expr, i + 1)
argument() is only valid if Expr is an expr_kind::call expression.
argument()
expr_kind::call