Move cursor to the
amount-th previous tab stop.
Repeat the following procedure
Moves the cursor left until it reaches a column marked as tab stop (that is not the column the cursor started on) or the left-most column of the screen.
If cursor origin is set and after this move the cursor is left of the left-most column of the scrolling region, move the cursor to the left-most column of the scrolling region.
See also horizontal tab for information about tab stop setup.