{{ $tocLevels := default (default 6 .Site.Params.BookToC) .Params.BookToC }}
{{ if and $tocLevels .Page.TableOfContents }}
  <aside class="book-toc levels-{{$tocLevels}} fixed">
    {{ with .Page.TableOfContents }}
    <label id="toc-control" for="toc-control">
    {{ . }}
    </label>
    {{ end }}
  </aside>
{{ end }}