include_extend.rb

module M
  def asdf
    p 'asdf'
  end
end

module M_
  extend M
end

M_.asdf #=> asdf

module M__
  extend M
  asdf # same of self.asdf
  
  def M__.ghjk
    p 'ghjk'
  end
  
  M__.ghjk
end

class C
  include M
end

C.new.asdf #=> asdf