anchor.diff:      <% end %>
       <p><%= link_to_add_fields "Add Phone", person_form, :phones %></p>
     </ul>
  +  <% if @person.new_record? %>
  +  <p>Save the person to enable setting children and parents.</p>
  +  <% end %>
     <h3>Children</h3>
     <ul>
       <%= person_form.collection_check_boxes(:child_ids,
  
positive.code:                                          :id,
                                           :name) do |box| %>
      <li>
        <%= box.check_box(disabled: @person.and_descendants.include?(box.object)).concat(box.label) %>
      </li>
      <% end %>
    </ul>
  
negative.code:       :id,
        :name) do |box| %>
        <li>
          <%= box.check_box(
                    disabled: @person.and_ancestors.include?(box.object) ||
                                @person.new_record?
                  ).concat(box.label) %>
        </li>
      <% end %>
    </ul>
  