public void objectRetracted(ObjectRetractedEvent ev) {
if ( ev.getOldObject() instanceof AbstractCellValue ) {
AbstractCellValue cellValue = (AbstractCellValue) ev.getOldObject();
removeCellValue( cellValue );
fireCellUpdatedEvent( new SudokuGridEvent( this,
cellValue.getRow(),
cellValue.getCol(),
cellValue.getValue() ) );
}
}