synchronized (mutex) {
        return sortedMap(delegate().headMap(toKey), mutex);
      }
    }

    @Override
    public K lastKey() {
      synchronized (mutex) {
        return delegate().lastKey();
      }
    }

    @Override
    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      synchronized (mutex) {

      synchronized (mutex) {
        return sortedMap(delegate().headMap(toKey), mutex);
      }
    }

    @Override
    public K lastKey() {
      synchronized (mutex) {
        return delegate().lastKey();
      }
    }

    @Override
    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      synchronized (mutex) {

  @Override
  public Comparator<? super K> comparator() {
    return keySet().comparator();
  }

  @Override
  public K firstKey() {
    return keySet().first();
  }

  @Override
  public K lastKey() {
    return keySet().last();
  }

  private ImmutableSortedMap<K, V> getSubMap(int fromIndex, int toIndex) {
    if (fromIndex == 0 && toIndex == size()) {
      return this;

  @Override
  public Comparator<? super K> comparator() {
    return keySet().comparator();
  }

  @Override
  public K firstKey() {
    return keySet().first();
  }

  @Override
  public K lastKey() {
    return keySet().last();
  }

  private ImmutableSortedMap<K, V> getSubMap(int fromIndex, int toIndex) {
    if (fromIndex == 0 && toIndex == size()) {
      return this;

            var dayOfWeek = firstDay.day();

            //initialize a 6 rows x 7 columns array for the calendar
            var calendar = [];
            calendar.firstDay = firstDay;
            calendar.lastDay = lastDay;

            for (var i = 0; i < 6; i++) {
                calendar[i] = [];
            }

            //populate the calendar with date objects