def _on_undo ( self, info ):
        if info.ui.history is not None:
            info.ui.history.undo()

    #---------------------------------------------------------------------------
    #  Handles a 'Redo' change request:
    #---------------------------------------------------------------------------
Handles an "Undo" change request.
dataset/ETHPy150Open enthought/traitsui/traitsui/handler.py/Handler._on_undo