Rubyでクラスメソッドの定義方法色々 ref: http://qiita.com/items/951

file0.rb

class A
  def A.class_method_one; end  def self.class_method_two; end  class << A
    def class_method_three; end
  end  class << self
    def class_method_four; end
  end
enddef A.class_method_five; endclass << A
  def class_method_six; end
end