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Solution to the Problem

The diagramed position occured in a training match between Mona and YL.
After YL narrowly missed a win earlier in the game, Mona found a long
forcing line, leading to a mate in 11 moves (21-ply).

The solution is presented move by move, so that you can treat it as a
series of easier problems, until you're able to find the mate.  Only
the toughest line of defence is given, so you'll have to work out the
details for yourself.

Mona vs YL    (Game 4, second training match,  March 2000)

move by move:

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . W B . . . |
5 | . W . . W W W . |
4 | . . W B B . . . |
3 | . . . . . . . . |
2 | W . . . . . . . |
1 | . . . B . . . . |
+-----------------+
a b c d e f g h

1. e4-d5 a2-a3

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . W B . . . |
5 | . W . B W W W . |
4 | . . W B . . . . |
3 | W . . . . . . . |
2 | . . . . . . . . |
1 | . . . B . . . . |
+-----------------+
a b c d e f g h

2. d4-c5 e5-d4

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . W B . . . |
5 | . W B B . W W . |
4 | . . W W . . . . |
3 | W . . . . . . . |
2 | . . . . . . . . |
1 | . . . B . . . . |
+-----------------+
a b c d e f g h

3. d1-c2 b5-d3

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . W B . . . |
5 | . . B B . W W . |
4 | . . W W . . . . |
3 | W . . W . . . . |
2 | . . B . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

4. c2-b3 d4-b4

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . W B . . . |
5 | . . B B . W W . |
4 | . W W . . . . . |
3 | W B . W . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

5. b3-a4 d3-b5

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . W B . . . |
5 | . W B B . W W . |
4 | B W W . . . . . |
3 | W . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

6. a4-a6 f5-f6

+-----------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | B . . W B W . . |
5 | . W B B . . W . |
4 | . W W . . . . . |
3 | W . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

7. a6-b7 f6-d8

+-----------------+
8 | . . . W . . . . |
7 | . B . . . . . . |
6 | . . . W B . . . |
5 | . W B B . . W . |
4 | . W W . . . . . |
3 | W . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

8. e6-d7 a3-a4

+-----------------+
8 | . . . W . . . . |
7 | . B . B . . . . |
6 | . . . W . . . . |
5 | . W B B . . W . |
4 | W W W . . . . . |
3 | . . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

9. b7-a6 d6-b6

+-----------------+
8 | . . . W . . . . |
7 | . . . B . . . . |
6 | B W . . . . . . |
5 | . W B B . . W . |
4 | W W W . . . . . |
3 | . . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

10. d5-c6 b4-b7

+-----------------+
8 | . . . W . . . . |
7 | . W . B . . . . |
6 | B W B . . . . . |
5 | . W B . . . W . |
4 | W . W . . . . . |
3 | . . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+-----------------+
a b c d e f g h

11. d7:b7++ 1-0

```