gtk:text-buffer-max-undo-levels (buffer)

buffer -- a gtk:text-buffer object
levels -- an unsigned integer with the number of undo levels

The gtk:text-buffer-max-undo-levels function gets the maximum number of undo levels to perform. The (setf gtk:text-buffer-max-undo-levels) function sets the maximum number. If 0, unlimited undo actions may be performed. Note that this may have a memory usage impact as it requires storing an additional copy of the inserted or removed text within the text buffer.