module Fatty

  def self.included(base)
    base.extend(self)
  end

  def barr
    bazz
  end

  private

  def bazz
    puts "fat!!!"
  end
end

class Foo
  def self.bar
    baz
  end

  include Fatty

  private

  def self.baz
    puts "fat"
  end
end

Foo.bar #=> fat
Foo.baz #=> fat
Foo.barr #=> fat!!!
Foo.bazz #=> NoMethodError: private method ‘bazz’ called for Foo:Class