# any time a tombstone immediately preceeds a empty, it can be marked empty
[ $ _ ] -> [ _ _ ]
# any time you lookup a key, it can be swapped with a tombstone immediately preceeding it
[ $ A ] -> [ A $ ]
# (moving the tombstone closer to a empty that will destroy it)
# if you don't have iterators, you can also jump over other keys
[ $ B A ] -> [ A B $ ]
[ $ B C A ] -> [ A B C $ ] # etc
# (this will cause a iterator at A to yield B again, or at B to skip A)
How well this keeps the load factor down depends on how aggressively you look for tombstone disposal opportunities, but it does keep it down.
