<% parents = [] %>
<ul>
    <% @categories.each do |category| %>
        <% parents.each do |parent| %>
      <% if !category.is_descendant_of?(parent) %>
        <% active_parents.delete(parent) %>
        </ul></li>
      <% end %>
    <% end %>
        <li>
        <%= category.name %>
        <% if category.leaf? %>
        </li>
    <% else %>
      <% parents << category %>
      <ul>
    <% end %>
    <% end %>
</ul>