# File lib/facet/array/at_rand.rb, line 47
  def rand_index( number=nil, exclusive=true )
    unless number
      rand(size)
    else
      number = number.to_int
      if exclusive  
        idx = (0...size).to_a
        return idx if number >= size
        return idx.sort_by{rand}.slice(0,number)
      else
        a = []
        n.times{ a << rand(size) }
        a
      end
    end
  end