expand.rb

def expand(parent)
  parent.posts.each do |child|
    link = "/collections#{@engine.getPathFor(child)}/rubyfragment.html"
    $document.write <<-END
      <div><a href='#{link}'>#{child['name']}<a></div>
      #{child['description']}
      <div style='padding-left: 20px'>
    END
    expand child
    $document.write <<-END
      </div>
    END
  end
end

fragment.html

<!-- the "src" attribute here must be site-absolute -->
<script type="server-parsed/ruby" src="/ruby/nav.rb"/>

nav.rb

require 'expand'

@engine = $GoGoEgo.getPlugin 'Posts'
expand @engine.current