If you wish to undo the operation last carried out in the program and make the pieces go back to the shape and state they had before the operation, click on Undo or press Ctrl+Z. The program stores in its memory and can undo a given number of operations carried out. You can select, from the list, the step up to which the program should cancel the operations carried out.
Note! You can set the maximum number of steps the program must memorize from Settings/ Files&Backup.,