/**
  * Note that this always returns null because the set is no longer valid after this call
  *
  * @param key
  * @return null
  */
 @Override
 public Set<V> removeAll(@NotNull Object key) {
   SpillableSetImpl<V> spillableSet = getHelper((K) key);
   if (spillableSet != null) {
     cache.remove((K) key);
     map.put((K) key, new ImmutablePair<>(0, spillableSet.getHead()));
     spillableSet.clear();
     removedSets.add(spillableSet);
   }
   return null;
 }