scratch.rb

# The goal is to make classes who include Foo also get the methods 
# from Bar, but after Foo has already been included

module Foo
  def foo; end
end

class FooBar
  include Foo
end

module Bar
  def bar; end
end

module Foo
  include Bar
end

FooBar.new.foo # => nil
FooBar.new.bar # => NoMethodError