Custom FormBuilder problems

I'm trying to create my first custom FormBuilder and I'm having a bit
of a problem. I'm trying to add a 'field_group' method to my builder
that takes a block, but when I yield to the block I seem to be getting
duplicate form field entries being produced. Maybe my code will make
this clearer:

class LabelledBuilder < ActionView::Helpers::FormBuilder
  def self.define_labelled_field_method(method_name)
    src = <<-end_src
     def #{method_name}(label, method, options = {})
       hint_text = options[:hint]
       @template.content_tag("tr") do
         @template.content_tag("td",
           @template.content_tag("label",
label.to_s.humanize.titleize, :for => method)) +
         @template.content_tag("td", super(method, options) +
           (hint_text.nil? ? '' : @template.content_tag("div",
hint_text, :class => "field_hint")))
       end
     end
   end_src
   class_eval src, __FILE__, __LINE__
  end

  def check_box(label, method, options = {})
    super(method, options) +
      @template.content_tag("label", label, :for => method)
  end

  def field_group(label, &proc)
    raise ArgumentError, "Missing block" unless block_given?
    @template.concat(yield, proc.binding) # <--- !!!! PROBLEM
SEEMS TO BE CAUSED BY THIS YIELD !!!
  end

  (field_helpers - %w(check_box)).each do |name|
    define_labelled_field_method(name)
  end
end

If I put this in my view

<% form_for :user, :url => {:controller => :user, :action
=> :new }, :builder => LabelledBuilder do |f| %>
  <table>
  <%= f.text_field "user name:", :user_name, :hint => "Enter user
name" %>
  <% f.field_group "includes:" do %>
    <%= f.check_box "box", :terms_of_service %>
  <% end %>
  </table>
<% end %>

the text field and the checkbox are emitted correctly, but then I also
get an inner <form> element embedded in the outer <form> element, and
the two <input> fields are repeated. Something like this:

<form action="/user/new" method="post">
  <table>
  <tr><td><label for="user_name">User Name:</label></td><td><input
hint="Enter user name" id="user_user_name" name="user[user_name]"
size="30" type="text" /><div class="field_hint">Enter user name</div></

</tr>

    <input id="user_terms_of_service" name="user[terms_of_service]"
type="checkbox" value="1" /><input name="user[terms_of_service]"
type="hidden" value="0" /><label for="terms_of_service">box</label>

<form action="/user/new" method="post">
  <table>

  <tr><td><label for="user_name">User Name:</label></td><td><input
hint="Enter user name" id="user_user_name" name="user[user_name]"
size="30" type="text" /><div class="field_hint">Enter user name</div></

</tr>

    <input id="user_terms_of_service" name="user[terms_of_service]"
type="checkbox" value="1" /><input name="user[terms_of_service]"
type="hidden" value="0" /><label for="terms_of_service">box</label>

  </table>
</form>

I believe it has something to do with the call to 'yield' in my
field_group method, but I'm not sure why this is happening. Can
anybody explain this to me?

TIA,
-- bb

I think I figured it out. The problem was related to the fact that I
was calling 'yield' within a call to @template.concat, and the block
contained <%= %> blocks, so the erb blocks were omitting the fields
and so was the call to @template.concat. Seems to be working now.

Thanks anyway,
-- bb