[1,2,3,1,2] is invalid because the substring [1,2] appears twice.
[1,2,1] is invalid because the substring [1,2] appears twice (once forward and once backward).
a(1) = 1 via [1];
a(2) = 2 via [1,1];
a(3) = 4 via [1,1,2];
a(4) = 6 via [1,1,2,2];
a(5) = 8 via [1,1,2,3,1];
a(6) = 10 via [1,1,2,2,3,1];
a(7) = 13 via [1,1,2,2,3,3,1];
a(8) = 16 via [1,1,2,2,3,1,4,2];
a(9) = 19 via [1,1,2,2,3,3,1,4,2];
a(10) = 22 via [1,1,2,2,3,1,4,2,5,1];
a(11) = 25 via [1,1,2,2,3,3,1,4,2,5,1];
a(12) = 29 via [1,1,2,2,3,3,1,4,4,2,5,1].
