How can I add UNDO functionality here

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Hello, I am creating a GUI which enables user to draw a line on image using imfreehand and the position points pixels value will be assigned to 0 so I am being able to remove overlappings of white objects on black background by hand.I want to add UNDO functionality to my program.Here my callback code is callback of TOGGLE BUTTON. I want user to cancel the last drawing (not all the things done by toggle,only last stage of while loop).How can I achieve this ? thank you for your opinions

global I
axes(handles.axes1);
while get(hObject,'value')
   h=imfreehand;
   setClosed(h,'false');
   pos=h.getPosition();
   pos=round(pos);
   s=numel(pos)/2;
   for p=1:s
       I(pos(p+s),pos(p))=0;
   end;
   imshow(I);
end;