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();