def subscript(self):
        """
        |True| if `w:vertAlign/@w:val` is 'subscript'. |False| if
        `w:vertAlign/@w:val` contains any other value. |None| if
        `w:vertAlign` is not present.
        """
        vertAlign = self.vertAlign
        if vertAlign is None:
            return None
        if vertAlign.val == ST_VerticalAlignRun.SUBSCRIPT:
            return True
        return False
|True| if `w:vertAlign/@w:val` is 'subscript'. |False| if
        `w:vertAlign/@w:val` contains any other value. |None| if
        `w:vertAlign` is not present.