can't subclass XML::Document?

[yermej]

I'm working with libxml and I can't seem to define a subclass of
XML:ocument. For example,

require 'libxml'

class SubDocument < XML:ocument
def initialize(name)
super
@sub_name = name
end

def sub_method
puts "the method"
end
end

s = SubDocument.new('test')

s.respond_to? 'sub_method'

=> false

s.class

=> XML:ocument

As you can see from the above, it's as if I'm aliasing XML:ocument,
rather than subclassing it. I have also tried including XML and then
defining the subclass as well as opening the XML module and then
defining the subclass -- same results in both cases. I've never come
across this before. Is there a way to fix this or should I just write
my class to contain an XML:ocument?

Thanks,
Jeremy