Let \(U = \{1,2,\ldots,n\}\) with \(n > 1000\).
Let \(k < n\) be a positive integer.
Let \(A,B \subseteq U\) with \(|A| = |B| = k\) and \(A \cap B = \varnothing\).
A permutation of \(U\) separates \(A\) from \(B\) if either all members of \(A\) appear before any member of \(B\), or all members of \(B\) appear before any member of \(A\).
How many permutations of \(U\) separate \(A\) from \(B\)?