This puzzle is Copyright © 2007 by James Dow Allen
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A (1-0) near upper left is impossible -- a duplicate (1-0) would be forced in the corner. A nearby (1-1) is impossible -- it would force a duplicate (1-1) adjacent.
The only possible (6-6)'s involve 6*, so it isn't (6*-2). The only possible (3-3)'s involve 3*, so it isn't (3*-5).
The two 3!'s connect only to 0 and 4, so the (3-0) and (3-4) are associated with them, and the remaining (3-0) and (3-4) are impossible.
This means the 3 at right middle is in a (3-5),
so the (3-5) at far left is impossible: We've located our first domino.
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With (3-2) located, and (5-3) partially located, we eliminate other (3-2)'s and (5-3)'s.
The two 6*'s can connect only to 0 and 4, so the (6-0) and (6-4) are associated with them, and the remaining (6-0)'s and (6-4)'s are impossible.
The two 5*'s can connect only to 1 and 5, so the (5-1) and (5-5) are associated with them, and the remaining (5-5) is impossible.
Finally, the 0* cannot be in (5-0) -- there'd be a (2-0) above it
and another one below (to avoid duplicating (5-0)).
This forces a (0-0) domino, preventing the (0-0) near upper left,
and leading to several forces shown in the next diagram.
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The forced (1-0) prevents other (1-0)'s, and leads to a series of simple forces.
With (6-1) and (2-2) located, their alternate sites can be eliminated.
Several simple forces now exist in the lower right as shown next.
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We continue the simple forces.
With (6-5), (2-1) located, their alternate sites can be eliminated.
By now, (5-4) and (6-2) have become single-siters, but having gotten this far without any use of the single-site deduction, let's not start now!
The (5-0) and (6-2) near upper right are forced simply.
Other (5-0) sites can be eliminated. This leads to location of (5-2) and (3-0), so their alternate sites are also eliminated. Complete solution quickly follows.
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This puzzle is Copyright © 2007 by James Dow Allen