This genre was developed by Palmer Mebane.
Draw a single closed loop through the grid by connecting the centres of cells. The loop can't go through any cells with a bold black border. If such a cell is coloured black, it will be outside the loop; if such a cell is coloured white, it will be inside the loop. Some cells will have numbers and arrows. A number with a horizontal arrow indicates the total length of horizontal line segments in the direction of the arrow. A number with a vertical arrow indicates the total length of vertical line segments in the direction of the arrow.