code: def formula_fields_for(record_or_name_or_array, *args, &block)
        options = args.extract_options!
        options[:builder] ||= @@builder
        fields_for(record_or_name_or_array, *(args << options), &block)
      end
docs: Generates a wrapper around fields_for with :builder set to FormulaFormBuilder.
  
   Supports:
  
   * f.formula_fields_for(@user.company)
   * f.fieldsula_for(@user.company)
  
   Equivalent:
  
   * f.fields_for(@user.company, :builder => Formula::FormulaFormBuilder))
  
   Usage:
  
     <% f.formula_fields_for(@user.company) do |company_f| %>
       <%= company_f.input :url %>
       <%= company_f.input :phone %>
     <% end %>