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

##</code><code>##

A = 2^0 = 1
B = 2^1 = 2
C = 2^2 = 4
D = 2^3 = 8

A C D = 1 + 4 + 8 = 13