if (wholeRow == null || (wholeRow.isEmpty() && backingMap.containsKey(rowKey))) {
        wholeRow = (SortedMap<C, V>) backingMap.get(rowKey);
      }
    }

    @Override
    @Nullable SortedMap<C, V> computeBackingRowMap() {
      updateWholeRowField();
      SortedMap<C, V> map = wholeRow;
      if (map != null) {
        if (lowerBound != null) {
          map = map.tailMap(lowerBound);
        }

    final void updateBackingRowMapField() {
      if (backingRowMap == null || (backingRowMap.isEmpty() && backingMap.containsKey(rowKey))) {
        backingRowMap = computeBackingRowMap();
      }
    }

    @Nullable Map<C, V> computeBackingRowMap() {
      return backingMap.get(rowKey);
    }

    // Call this every time we perform a removal.
    void maintainEmptyInvariant() {
      updateBackingRowMapField();

      if (wholeRow == null || (wholeRow.isEmpty() && backingMap.containsKey(rowKey))) {
        wholeRow = (SortedMap<C, V>) backingMap.get(rowKey);
      }
    }

    @Override
    @Nullable SortedMap<C, V> computeBackingRowMap() {
      updateWholeRowField();
      SortedMap<C, V> map = wholeRow;
      if (map != null) {
        if (lowerBound != null) {
          map = map.tailMap(lowerBound);
        }