namespace std {
 
// execution policy type trait:
template<class T> struct is_execution_policy;
template<class T> inline constexpr bool is_execution_policy_v 
  = is_execution_policy<T>::value;
 
namespace execution { 
 
// sequential execution policy:
class sequenced_policy;
 
// parallel execution policy:
class parallel_policy;
class parallel_unsequenced_policy;
 
// execution policy objects:
inline constexpr sequenced_policy seq{ /*unspecified*/ };
inline constexpr parallel_policy par{ /*unspecified*/ };
inline constexpr parallel_unsequenced_policy par_unseq{ /*unspecified*/ };
 
}
 
}