<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def mwl(self, event):
 """Mouse Wheel - under tkinter we seem to need Tk v8.5+ for this """

        if event.num == 4: # up on Linux
            self.top.f.canvas.yview_scroll(-1*self._tmwm, 'units')
        elif event.num == 5: # down on Linux
            self.top.f.canvas.yview_scroll(1*self._tmwm, 'units')
        else: # assume event.delta has the direction, but reversed sign
            self.top.f.canvas.yview_scroll(-(event.delta)*self._tmwm, 'units')