boost::proto::result_of::left — A metafunction that returns the type of the left child of a binary Proto expression.
// In header: <boost/proto/traits.hpp> template<typename Expr> struct left : { };
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>