new chapter

SUMMARY.md

@@ -34,7 +34,7 @@
 	* [Races](races.md)
 	* [Send and Sync](send-and-sync.md)
 	* [Atomics](atomics.md)
-* [Example: Implementing Vec](vec.md)
+* [Implementing Vec](vec.md)
 	* [Layout](vec-layout.md)
 	* [Allocating](vec-alloc.md)
 	* [Push and Pop](vec-push-pop.md)
@@ -43,4 +43,5 @@
 	* [Insert and Remove](vec-insert-remove.md)
 	* [IntoIter](vec-into-iter.md)
 	* [Drain](vec-drain.md)
-	* [Final Code](vec-final.md)
+	* [Final Code](vec-final.md)
+* [Implementing Arc and Mutex](arc-and-mutex.md)

atomics.md

@@ -220,8 +220,5 @@ relaxed operations can be cheaper on weakly-ordered platforms.
 
 
 
-TODO: implementing Arc example (why does Drop need the trailing barrier?)
-
-
 [C11-busted]: http://plv.mpi-sws.org/c11comp/popl15.pdf
 [C11-model]: http://en.cppreference.com/w/c/atomic/memory_order