module A
end

module B
end

module A::A1
  module_function
  def test
    puts "hola"
  end
end

module A::A2
  module_function
  def test2
    A1.test
  end
end

module B
  module B1
    module_function
    def test
      puts "bolas"
    end
  end
end

module B
  module B2
    module_function
    def test2
      B1.test
    end
  end
end

B::B2.test2 # => "bolas"
A::A2.test2 # => NameError: uninitialized constant A::A2::A1