add Chapter18

Chapter18/cib/libs/stdx/priority.hpp

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <cstddef>
+
+namespace stdx {
+inline namespace v1 {
+template <std::size_t N> struct priority_t : priority_t<N - 1> {};
+template <> struct priority_t<0> {};
+
+template <std::size_t N> constexpr inline auto priority = priority_t<N>{};
+} // namespace v1
+} // namespace stdx