-/* We introduce new member `tab_offset'. We need it because of the
- existence of wide-column characters. There is a case that the
- line-break occurs at a wide-column character and the number of
- columns of the line gets less than width.
-
- Example (where W_ stands for a wide-column character):
- ----------
- abcdefgh\\
- W_
- ----------
-
- To handle this case, we should not calculate the tab offset by
- tab_offset += width;
-
- Instead, we must remember tab_offset of the line.
-
- */
-