_submenu.html.erb

<% children = @page.root.children.map {|child| (child.in_menu?) ? child : nil}
if children.any?
	unless defined?(selected_item)
      selected_item = collection.detect{|page| selected_page?(page)}
      selected_item = @page if selected_item.nil?
    end
    sibling_count = children.size - 1
     -%>
<ul id="submenu"><%= render :partial => "/shared/submenu_branch", :collection => children, :locals => {
   :hide_children => hide_children,
   :sibling_count => sibling_count,
   :collection => collection,
   :selected_item => selected_item,
   :apply_css => true #if you don't care about class='first' class='last' or class='selected' set apply_css to false for speed.
 }
 -%></ul><% end -%>

_submenu_branch.html.erb

<% if submenu_branch && submenu_branch.in_menu? then
	classes = css_for_menu_branch(submenu_branch, submenu_branch_counter, sibling_count||=nil, collection, selected_item ||= nil) -%><li<%= " class='#{classes.join(' ')}'".html_safe if classes.any? -%>><%= link_to submenu_branch.title, submenu_branch.url -%>
<%
# logger.info "#{submenu_branch.title} children.any?:#{submenu_branch.children.any?} selected_page?: #{selected_page?(submenu_branch)} descendant_page_selected?:#{descendant_page_selected?(submenu_branch)}\n#{request.path.encoding}"
%>
	<% if classes.include?('selected') && (children = submenu_branch.children).any? %>
<ul><%= render :partial => "/shared/submenu_branch",
               :collection => children,
               :locals => {
                 :apply_css => local_assigns[:apply_css],
                 :hide_children => hide_children,
                 :collection => children,
                 :selected_item => selected_item,
                 :sibling_count => children.size - 1
               } -%></ul><% end -%></li>
<% end -%>