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A B C D Size 4 = ABCD rank 1 Size 3 = ABC ABD ACD BCD rank 1 .. 4 Size 2 = AB AC AD BC BD CD rank 1 .. 6 Size 1 = A B C D rank 1 .. 4
A = 2^0 = 1 B = 2^1 = 2 ... D = 2^3 = 8 A C D = 1 + 4 + 8 = 13