Define a set to be \(\textbf{selfish}\) if the number of elements it has is in the set. For example, \(X_{10}=\{1,\ 2,\ 3,\ 4,\ 5,\ 6,\ 7,\ 8,\ 9,\ 10\}\) is selfish because it has 10 elements, and 10 is in the set. How many selfish subsets does Xn have, where n is a positive integer?
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