where's Module#append_features ???

We all know that Class is inherited from Module in Ruby,
so we(or I in fact) would expect Module#append_features would
be available in Class' instance as well as Module's instance.

i.e.,

class C; end
C.respond_to? :append_features, true # => true

In MRI 1.8/1.9 and JRuby, the answer is `false'.
I know perhaps Class#append_features was meaningless in Ruby,
but it confuses me a lot. I start wondering if append_features
was defined in each (instance of Module)'s metaclass?

I don't know.

In Rubinius, yes, Class#append_features exists anyway.
That's what I expected.

I think JRuby tries to be very compatible with MRI, and Rubinius
favors consistency, soundness, etc. over compatibility.
I like the later.

==
I also start to think that Class < Module somehow
violates LSP. You can include module, but not class.