Author: Andrey Bogdanov
Draw a single continuous loop in the grid using horizontal and vertical line segments so that the loop visits every square exactly once. It should not cross or overlap itself. It makes a 90-deg turn at every square with a circle. There is exactly one turn between two consecutive circles that the loop visits.
Grid control: To draw a line press the mouse button at a start cell and move mouse to another cell holding the button. More details about grid control are at the help page.