ruby permutation
a = [1, 2, 3]
a.permutation.to_a #=> [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
a.permutation(1).to_a #=> [[1],[2],[3]]
a.permutation(2).to_a #=> [[1,2],[1,3],[2,1],[2,3],[3,1],[3,2]]
a.permutation(3).to_a #=> [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
a.permutation(0).to_a #=> [[]] # one permutation of length 0
a.permutation(4).to_a #=> [] # no permutations of length 4