Experiments with where Module#prepend locates the module in the ancestors chain.

Ruby 2.0 Module#prepend

[ruby-2.0.0dev] code/ruby $ irb
irb2.0.0> module Bar; def who; puts 'bar'; end; end
#2.0.0 => nil
irb2.0.0> class Baz; def who; puts 'baz'; end; end
#2.0.0 => nil
irb2.0.0> class Foo < Baz; def who; puts 'foo'; end; end
#2.0.0 => nil
irb2.0.0> Foo.ancestors
#2.0.0 => [Foo, Baz, Object, Kernel, BasicObject]

All normal so far...

irb2.0.0> module Quux; def who; puts 'quux';end ;end
#2.0.0 => nil
irb2.0.0> Foo.class_eval { prepend Quux }
#2.0.0 => Foo
irb2.0.0> Foo.ancestors
#2.0.0 => [Quux, Foo, Baz, Object, Kernel, BasicObject]

While an include Quux would have placed the module after Foo and before the superclass Baz, prepend puts it at the front of the list.

irb2.0.0> Foo.class_eval { include Bar }
#2.0.0 => Foo
irb2.0.0> Foo.ancestors
#2.0.0 => [Quux, Foo, Bar, Baz, Object, Kernel, BasicObject]

Here we see that an included module, Bar, does still go there in fact.

irb2.0.0> Foo.new.who
quux
#2.0.0 => nil
irb2.0.0> class Kid < Foo; def who; puts 'Kid'; end; end
#2.0.0 => nil
irb2.0.0> Kid.ancestors
#2.0.0 => [Kid, Quux, Foo, Bar, Baz, Object, Kernel, BasicObject]

A new subclass has all of its superclass's ancestors.

irb2.0.0> module Late; def who; puts 'late'; end; end
#2.0.0 => nil
irb2.0.0> Foo.class_eval { prepend Late }
#2.0.0 => Foo
irb2.0.0> Foo.ancestors
#2.0.0 => [Late, Quux, Foo, Bar, Baz, Object, Kernel, BasicObject]

A second prepend on Foo again places the module at the beginning of its ancestors list.

irb2.0.0> Kid.ancestors
#2.0.0 => [Kid, Late, Quux, Foo, Bar, Baz, Object, Kernel, BasicObject]

And the subclass has a consistent behavior; the superclass's ancestors follow in the list.