namespace std {
 
// execution policy type trait:
template<class T> struct is_execution_policy;
template<class T> 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_p_policy;
 
// execution policy objects:
constexpr sequenced_policy seq{ /*unspecified*/ };
constexpr parallel_policy par{ /*unspecified*/ };
constexpr parallel_unsequenced_p_policy par_unseq{ /*unspecified*/ };
 
}
 
}