# File lib/backports/1.9.1/stdlib/prime.rb, line 34
  def prime?
    return self >= 2 if self <= 3
    return false if self % 2 == 0 or self % 3 == 0
    (5..(self**0.5).floor).step(6).each do |i|
      if self % i == 0 || self % (i + 2) == 0
        return false
      end
    end
    true
  end