'|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.'

@property
def subscript(self):

    vertAlign = self.vertAlign
    if (vertAlign is None):
        return None
    if (vertAlign.val == ST_VerticalAlignRun.SUBSCRIPT):
        return True
    return False
