Place 4 watchmen ⚇ so that they can see every box in the puzzle grid,
including the mirrors.
Watchmen can see every box in the horizontal and vertical
directions, up to a mirror, a watchman, or the edge of the grid.
When there is a mirror ╲ or ╱ it will turn the line of sight
either right or left, according to its angle.
In this example, the watchman in the upper left corner can use the two mirrors
to see the watchman at the center of the bottom row.