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  • vanjor thinks this is interesting:

{(1, 2), (1, 3), (1, 4), (1, 5), (3, 4), (3, 5)}

From

Cover of Learning Spark

Note

why not {(1, 2), (1, 3), (1, 4), (1, 5), (3, 4), (3, 5),(3, 4), (3, 5)} ,there should be another (3, 4), (3, 5)? though I have already checked, there is not another (3, 4), (3, 5), but why