template<class T> struct is_execution_policy { see below };
class execution::sequenced_policy { unspecified };
class execution::parallel_policy { unspecified };
class execution::parallel_unsequenced_policy { unspecified };
class execution::unsequenced_policy { unspecified };
inline constexpr execution::sequenced_policy execution::seq{ unspecified };
inline constexpr execution::parallel_policy execution::par{ unspecified };
inline constexpr execution::parallel_unsequenced_policy execution::par_unseq{ unspecified };
inline constexpr execution::unsequenced_policy execution::unseq{ unspecified };