std::experimental::parallel::seq, std::experimental::parallel::par, std::experimental::parallel::par_vec

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Technical specifications
Filesystem library (filesystem TS)
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Library fundamentals 2 (library fundamentals TS v2)
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Extensions for parallelism (parallelism TS)
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constexpr sequential_execution_policy seq{};
(parallelism TS)
constexpr parallel_execution_policy par{};
(parallelism TS)
constexpr parallel_vector_execution_policy par_vec{};
(parallelism TS)

seq, par and par_vec are instances of the execution policy types sequential_execution_policy, parallel_execution_policy and parallel_vector_execution_policy respectively. They are used to specify the execution policy of parallel algorithms - i.e., the kinds of parallelism allowed.