refinery_test_3.rb

class Foo
  def self.method_added(meth); puts "Foo#method_added: #{meth}"; end
  def self.singleton_method_added(meth); puts "Foo#singleton_method_added: #{meth}"; end
  
  # Singleton
  class << self
    def self.method_added(meth); puts "singleton#method_added: #{meth}"; end
    def self.singleton_method_added(meth); puts "singleton#singleton_method_added: #{meth}"; end
    
    # Metasingleton
    class << self
      def self.method_added(meth); puts "metasingleton#method_added: #{meth}"; end
      def self.singleton_method_added(meth); puts "metasingleton#singleton_method_added: #{meth}"; end
      
      # Metametasingleton
      class << self
        def self.method_added(meth); puts "metametasingleton#method_added: #{meth}"; end
        def self.singleton_method_added(meth); puts "metametasingleton#singleton_method_added: #{meth}"; end
        
      end
      
    end
    
  end
  
end

puts '--- -- --- ! --- -- ---'

class Foo
  def instance_method; end
  def self.singleton_method; end
  class << self
    def instance_method_on_singleton; end
    def self.singleton_method_on_singleton; end
  end
end

results

Foo#singleton_method_added: singleton_method_added
singleton#singleton_method_added: singleton_method_added
metasingleton#singleton_method_added: singleton_method_added
metametasingleton#singleton_method_added: singleton_method_added
--- -- --- ! --- -- ---
Foo#method_added: instance_method
Foo#singleton_method_added: singleton_method
Foo#singleton_method_added: instance_method_on_singleton
singleton#singleton_method_added: singleton_method_on_singleton