Traceback (innermost last):
  Module ZPublisher.Publish, line 119, in publish
  Module ZPublisher.mapply, line 88, in mapply
  Module ZPublisher.Publish, line 42, in call_object
  Module collective.dancing.browser.preview, line 67, in __call__
  Module collective.singing.scheduler, line 109, in render_message
  Module collective.dancing.composer, line 284, in render
  Module plone.memoize.volatile, line 272, in replacement
  Module collective.dancing.composer, line 274, in _render
  Module stoneagehtml.stoneagehtml, line 91, in compactify
  Module stoneagehtml.stoneagehtml, line 208, in compactify
  Module cssutils.css.cssstylesheet, line 104, in _setCssRules
AttributeError: 'list' object attribute 'append' is read-only