* enough it's not especially worth it.
   */
  abstract class Itr<T extends @Nullable Object> implements Iterator<T> {
    int entryIndex = backingMap.firstIndex();
    int toRemove = -1;
    int expectedModCount = backingMap.modCount;

    @ParametricNullness
    abstract T result(int entryIndex);

    private void checkForConcurrentModification() {