int entryIndex = backingMap.firstIndex();
    int toRemove = -1;
    int expectedModCount = backingMap.modCount;

    @ParametricNullness
    abstract T result(int entryIndex);

    private void checkForConcurrentModification() {
      if (backingMap.modCount != expectedModCount) {
        throw new ConcurrentModificationException();
      }
    }

    @Override