Fill the diagram with crosses and circles, such that no four equal symbols appear in a row. Rows can be horizontal, vertical, and diagonal.
If the middle square would contain a cross, there would be four in a row diagonally, so it has to be a circle. But now the square to the left of it can not also be a circle, or there would be four on a row horizontally, so that has to be a cross. Now on the right side of the upper row there can not be cross, so that becomes a circle.
On the second row there are already three crosses next to each other, so the next one is a circle. But now towards the lower left we need to avoid to get four circles in a row, so in the third square we need a cross. In the same way next to it we need a circle. In all remaining squares there is only one possibility left.
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