boost::proto::result_of::child — A metafunction that returns the type of the Nth child of a Proto expression, where N is an MPL Integral Constant.
N
template<typename Expr, typename N = mpl::long_<0> > struct child : proto::result_of::child_c<Expr, N::value> { };
proto::result_of::child<Expr, N> is equivalent to proto::result_of::child_c<Expr, N::value>.
proto::result_of::child<Expr, N>
proto::result_of::child_c<Expr, N::value>