Demonstrates how the behaviour of include and prepend on a singleton class differs between Ruby 2.0 and 2.1

singleton_class_prepend.rb

module TestModule
end

class Included
  class << self
    include TestModule
  end
end

class Prepended
  class << self
    prepend TestModule
  end
end

p Included.singleton_class.ancestors
p Prepended.singleton_class.ancestors

# Ruby 2.0:
# [TestModule, Class, Module, Object, Kernel, BasicObject]
# [TestModule, #<Class:Prepended>, Class, Module, Object, Kernel, BasicObject]

# Ruby 2.1:
# [#<Class:Included>, TestModule, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]
# [TestModule, #<Class:Prepended>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]