Ruby 2.1 In Detail(globaldev.co.uk)

    class User; def foo; "foo"; end; end
    # => :foo
    module M; def bar; "bar"; end; end
    # => :bar

    u = User.new
    # => #<User:0x007f2e1abc9e00>
    u.extend M
    # => #<User:0x007f2e1abc9e00>
    
    u.singleton_class.ancestors
    # => [#<Class:#<User:0x007f2e1abc9e00>>,
     M,
     User,
     Object,
     PP::ObjectMixin,
     Kernel,
     BasicObject]