Problem 4: One reason tombstones are rarely used is that each tombstone must persist to prevent the dangling pointer problem. We may be able to solve this problem by implementing garbage collection on the tombstones. If we do so, should we use reference counters or mark-and-sweep? Either argue that one method is superior to the other or argue that both methods are equally good (or equally poor). (Hint: why would a language need to use tombstones?)