BAB ruby class for OGL system

Jamis Buck's Dice class inspired me (a lot). A few weeks ago, I decided to start thinking about a role-playing session manager. I'm still thinking about it, but in the interim, I started work on a few classes, just to "feel better". Included below is a BAB class. It is released as OGC as described by the OGL. This class also has associated tests, if you are truly curious.

$ 5.bab.good 
=> 5

$ 4.bab.poor # Suck it wizard
=> 2

$ 3.bab.medium + 5.bab.poor + 3.bab.poor # 3 Rogue / 5 Wizard / 3 Arcane Trickster
=> 5

class Bab
  class InvalidCount < Exception # :nodoc:
  end
  TYPES = [:good, :medium, :poor]
  attr_reader :count, :epic_count

  def initialize( count )
    raise InvalidCount unless count.is_a?(Integer) && count > 0
    @count = count > 20 ? 20 : count
    @epic_count = count > 20 ? count - 20 : 0
  end

  def good
    self.count + epic_value    
  end

  def medium
    ((self.count * 3) / 4)  + epic_value
  end

  def poor
    (self.count / 2)  + epic_value
  end

  private
  def epic_value
    (self.epic_count + 1) / 2 
  end

end

class Integer
  def bab
    Bab.new(self)
  end
end