"are encouraged" -> "should"
Elimnate #include <type_traits>
Add semicolon after vector_execution_policy
"The behavior of programs that add specializations for is_execution_policy is undefined."
copied from meta.type.synop p1.
"implementation-defined non-standard" -> "implementation-defined"
Adapt wording for sequential_execution_policy, parallel_execution_policy, and vector_execution_policy from piecewise_construct_t.
Adopt consistent verbiage about is_execution_policy in
execution_policy's specification.