The class unsequenced_policy is an execution policy type
used as a unique type to disambiguate parallel algorithm overloading and
indicate that a parallel algorithm's execution may be vectorized,
e.g., executed on a single thread using instructions
that operate on multiple data items.
During the execution of a parallel algorithm with
the execution::unsequenced_policy policy,
if the invocation of an element access function exits via an uncaught exception,
terminate() shall be called.