boost::proto::result_of::left — A metafunction that returns the type of the left child of a binary Proto expression.
template<typename Expr> struct left : proto::result_of::child_c< Expr, 0 > { };
proto::result_of::left<Expr> is equivalent to proto::result_of::child_c<Expr, 0>.
proto::result_of::left<Expr>
proto::result_of::child_c<Expr, 0>