how to use LabellingFormBuilder

Railsers:

As usual for a product written very rapidly, most of its documentation
assumes someone else told you something critical.

For example, the file form_helper.rb tempts me with this documentation:

  # <% form_for :person, @person, :url => { :action => "update" },
          :builder => LabellingFormBuilder do |f| %>

That's beautiful. Now what do I require() into my controller.rb file
to get that LabellingFormBuilder identifier? Poking around with
'form_helper', 'helper', etc doesn't do it.

Hello Phlip,