boost::yap::get
// In header: <boost/yap/algorithm.hpp> template<typename Expr, typename I> decltype(auto) constexpr get(Expr && expr, I const & i);
Forwards the i-th element of expr to the caller. If expr is a reference expression, the result is get(deref(expr), i).
get(deref(expr), i)
get() is only valid if Expr is an expression.
get()