Range
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You have a grid of squares; some squares contain numbers.
Your job is to colour some of the squares black, such that
several criteria are satisfied:

* no square with a number is coloured black.

* no two black squares are adjacent (horizontally or vertically).

* for any two white squares, there is a path between them using
only white squares.

* for each square with a number, that number denotes the number
of squares reachable from that square going in each direction until
hitting a wall or a black square.

Click with the left button to paint a square black, or with the right
button to mark a square with a dot to indicate that you are sure it
should not be painted black. 

