Chagall Reference   Delete Undo-buffer     12.5.93
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   This function deletes the Undo-buffer of the
   current picture, releasing a corresponding 
   amount of memory.

   At the next use of the 'Adopt' function, by 
   the way, the Undo-buffer will be automatically
   created again as long as sufficient memory is 
   available for this.

   Since deleting an Undo-buffer really releases
   memory, one can delete a picture's Undo-buffer
   to free memory for other purposes.

