def mid(self):
        """ Constructs the middle line of the element"""
        ret = self.mid_format % self.mid_content.center(
            self.width, self._mid_padding)
        if self.right_fill:
            ret = ret.ljust(self.right_fill, self._mid_padding)
        if self.left_fill:
            ret = ret.rjust(self.left_fill, self._mid_padding)
        ret = ret.center(self.layer_width, self.mid_bck)
        return ret
Constructs the middle line of the element