3 Draw a single closed loop by connecting together the centres of
4 adjacent grid squares, so that some squares end up as corners, some as
5 straights (horizontal or vertical), and some may be empty. Every
6 square containing a black circle must be a corner not connected
7 directly to another corner; every square containing a white circle
8 must be a straight which is connected to <em>at least one</em> corner.
10 Drag between squares to draw or undraw pieces of the loop.
11 Alternatively, left-click the edge between two squares to turn it on
12 or off. Right-click an edge to mark it with a cross indicating that
13 you are sure the loop does not go through it.