Split the grid into subrectangles, so that each subrectangle contains exactly one marked cell.
It is not allowed for four subrectangles to meet at a point.
This marked cell indicates how the dimensions of the subrectangle compare. If the subrectangle is longer than it is tall, the marked cell should be marked with a horizontal line, whereas if it is taller than it is long, it should be marked with a vertical line. Finally, if the subrectangle is square, the marked cell should be marked with a plus.