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EXAMPLE
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1 is not a prime,
1 + 3 is not a prime,
1 + 3 + 2 is not a prime,
1 + 3 + 2 + 4 is not a prime.
So [1, 3, 2, 4] satisfies all the conditions.
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a(1) = 1: [[1]];
a(3) = 1: [[1, 3, 2]];
a(4) = 3: [[1, 3, 2, 4], [1, 3, 4, 2], [4, 2, 3, 1]];
a(5) = 8: [[1, 3, 2, 4, 5], [1, 3, 4, 2, 5], [1, 5, 2, 4, 3], [1, 5, 4, 2, 3], [4, 2, 3, 1, 5], [4, 2, 3, 5, 1], [4, 5, 1, 2, 3], [4, 5, 3, 2, 1]].
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