code: private CssConstruct handleAtRuleParam(CssToken start, CssTokenIterator iter,
        CssContentHandler doc, CssErrorHandler err)
    {
      return CssConstructFactory.create(start, iter, MATCH_SEMI_OPENBRACE,
          ContextRestrictions.ATRULE_PARAM);
    }
docs: With inparam token being the first token of an atrule param, create the
  construct and return it.
  
  @return A CssConstruct, or null if fail.