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EXAMPLE
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a(10) = 2: [9,1], [1,9].
a(11) = 1: [1,9,1].
a(12) = 8: [9,2,1], [9,1,2], [2,9,1], [2,10], [2,1,9], [10,2], [1,9,2], [1,2,9].
a(13) = 12: [9,3,1], [9,1,3], [9,1,2,1], [3,9,1], [3,1,9], [2,1,9,1], [1,9,3], [1,9,2,1], [1,9,1,2], [1,3,9], [1,2,9,1], [1,2,1,9].
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