? mark is implemented. You can place a ? mark by right-clicking on a square with a flag. You can release it by continuing to right-click.
The type of hints indicating the location of mines can now be selected as either normal or subtractive type.
The normal type still indicates the number of mines in the surrounding squares.
The subtraction type indicates the number of mines in the surrounding square minus the number of flags in the surrounding square. If this value is 0 and there are still closed squares in the surrounding area, it is shown in black; if all the surrounding closed squares have flags, it is shown in gray.