    # File lib/facets/core/enumerable/permutation.rb, line 10
        def permutation(number)
            arr = to_a
            out = arr[0..0]
            nextfactor = factor = 1
            arr.each_with_index {|x,i|
                case i
                when 0: next
                else
                    nextfactor = factor * (i+1)
                    out.insert(number % nextfactor / factor, x)
                    factor = nextfactor
                end
            }
            out
        end