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This is the simple case, with a one-point eye for Black.

t = ( + ) - 1 = 5.

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Black's group has a larger eye. The -/-stones are needed to reduce the effective liberties in the position that has been used so far by one.

t = ( + ) - 1 = 4.

A larger eye favours Black (he needs one ko-threat less than with the one-point eye), so White should try to prevent Black from getting one. However, in the problem, White cannot reach this aim successfully.

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There is the second, and shared, ko-shape in the centre.

t = ( + ) - 1 = 4.

This is the same result as with the larger Black eye.