finally // finally(f) makes a final_action{f} with a destructor that invokes f
narrow_cast // narrow_cast<T>(x) is static_cast<T>(x)
narrow // narrow<T>(x) is static_cast<T>(x) if static_cast<T>(x) == x with no signedness promotions, or it throws narrowing_error (e.g., narrow<unsigned>(-42) throws)
[[implicit]] // "Marker" to put on single-argument constructors to explicitly make them non-explicit.
move_owner // p = move_owner(q) means p = q but ???
joining_thread // a RAII style version of std::thread that joins.
index // a type to use for all container and array indexing (currently an alias for ptrdiff_t)