# File lib/facets/core/enumerable/permutation.rb, line 29
    def permutation_number(original_array=self.to_a.sort)
        arr = to_a
        m = 1
        v = 0
        last_indicies = Hash.new(0)
        original_array.each_with_index {|x,i|
            next if i==0
            m *= i
            done = original_array[0..i]
            v += m * arr.select {|y| done.include?(y)}.rindex(x)
        }
        v
    end