anchor.diff:    </div>
   
     <div class="l-2col-main">
  +    <a id="top" />
  +
       <% contents.each_with_index do |child_category, i| %>
         <section class="category-detail category-detail--<%= i+1 %>">
           <h2 class="category-detail__heading" id="<%= child_category.id %>"><%= link_to child_category.title, child_category.path %></h2>
  
positive.code: <div class="l-2col">
    <div class="l-2col-side">
      <ul class="category-filter">
        <li class="category-filter__list-item is-selected"><%= link_to 'All', "##{contents.first.id}" %></li>
        <% contents.each do |child_category| %>
          <li class="category-filter__list-item"><%= link_to child_category.title, "##{child_category.id}" %></li>
        <% end %>
  
negative.code:             <% end %>
            </ol>
          <% end %>
  
          <a href="#top">Go to top</a>
        </section>
      <% end %>
    </div>
  