inc_ext.rb

#!/usr/bin/env ruby

module A
  def self.included(base)
    base.extend ClassMethods
  end

  def a_instance_method
    puts 'a instance method'
  end
    
  module ClassMethods
    def a_class_method
      puts 'a class method'
    end
  end
end

module B
  def self.included(base)
    base.send :include, A
    base.extend ClassMethods
  end

  def b_instance_method
    puts 'b instance method'
  end

  module ClassMethods
    def b_class_method
      puts 'b class method'
    end
  end
end

class Bar
  include B
end

class Foo < Bar; end

Bar.a_class_method
Bar.new.a_instance_method
Bar.b_class_method
Bar.new.b_instance_method