In the map of London Tower shown below, five guards are represented by the letters A, B, C, D, and E (in the grid). Promptly at the firing of a gun, which denotes the setting of the sun and the changing of the guard, guard A marches out by exit a, B exits at b, C exits at c, D exits at d, and guard E marches from his present cell to cell F.
The problem is to discover how the five guards can make these five marches without any one guard crossing the line of march of another. In other words, no more than one line of march is permitted through any one cell.
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