34auto keys(Map
const &m) -> std::vector<typename Map::key_type> {
35 using value_type =
typename Map::value_type;
39 std::vector<typename Map::key_type> ret(m.size());
41 std::transform(begin(m), end(m), ret.begin(),
42 [](value_type
const &kv) { return kv.first; });