<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def scroll_down(self):
 """ Returns the next command if any. """

        self._index -= 1
        if self._index < 0:
            self._index = -1
            return ''
        try:
            return self._history[self._index]
        except IndexError:
            return ''