def _column_pad_filter(self, next_filter):
        """ Expand blank lines caused from overflow of other columns to blank
        whitespace.  E.g.
            INPUT: [
                ["1a", "2a"],
                [None, "2b"],
                ["1b", "2c"],
                [None, "2d"]
            ]
            OUTPUT: [
                ["1a", "2a"],
                [<blank>, "2b"],
                ["1b", "2c"],
                [<blank>, "2d"]
            ]
        """
        next(next_filter)
        while True:
            line = list((yield))
            for i, col in enumerate(line):
                if col is None:
                    line[i] = self._get_blank_cell(i)
            next_filter.send(line)