code: @Nonnull
    @ReturnsMutableCopy
    public ICommonsList <CSSUnknownRule> getAllUnknownRules ()
    {
      return m_aRules.getAllMapped (r -> r instanceof CSSUnknownRule, r -> (CSSUnknownRule) r);
    }
docs: Get a list of all top-level rules that are unknown rules (implementing
  {@link CSSUnknownRule}).
  
  @return A copy of all contained unknown <code>@</code> rules. Never
  <code>null</code>.