Undo / Redo of the screen element :
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The function of the "Undo" is to restore the previous action.
If the user wants to retrieve the previous action,then click
on "Edit > Undo" or click on "Undo" button from the Standard
Toolbar and the previous action will be undone.
  
Keep on clicking "Edit > Undo" from the Menu bar or on "Undo" button
from the Standard Toolbar to restore the previous actions one by one.
  
The function of the "Redo" is to reverse the action of the "Undo"
command.If the user decides,not to undo an action, then click on 
"Edit > Redo" from the Menu bar or on "Redo" button
from the Standard Toolbar to Redo the action.