==> geometry/coloring/triominoes.s <==
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---***+*
---...+*
---*+O+*
---*+...
---*+***

There is only one way to remove a square, aside from rotations and
reflections.  To see that there is at most one way, do this:  Label
all the squares of the chessboard with A, B or C in sequence by rows
starting from the top:

		ABCABCAB
		CABCABCA
		BCABCABC
		ABCABCAB
		CABCABCA
		BCABCABC
		ABCABCAB
		CABCABCA

Every triomino must cover one A, one B and one C.  There is one extra
A square, so an A must be removed.  Now label the board again by
rows starting from the bottom:

		CABCABCA
		ABCABCAB
		BCABCABC
		CABCABCA
		ABCABCAB
		BCABCABC
		CABCABCA
		ABCABCAB

The square removed must still be an A.  The only squares that got
marked with A both times are these:

		........
		........
		..A..A..
		........
		........
		..A..A..
		........
		........