C++17: Execution Policy Types

C++17 has three execution policy types:

Class                                        Constant
-------------------------------------------  ---------
std::execution::sequenced_policy             std::execution::seq
std::execution::parallel_policy              std::execution::par
std::execution::parallel_unsequenced_policy  std::execution::par_unseq
  
These constants choose algorithm overloads with the corresponding policies.

"seq"       specifies that the algorithm must not be parallelized.
"par"       specifies that the algoritmm may      be parallelized.
"par_unseq" specifies that the algorithm may      be parallelized and
              in an unsequenced way.

References:
http://en.cppreference.com/w/cpp/algorithm/execution_policy_tag
http://en.cppreference.com/w/cpp/algorithm/execution_policy_tag_t