source.rb

require 'rubygems'
require 'nokogiri'

include Nokogiri::XML

b = Builder.new {
  fields {
    field '1'
    field '2'
  }
}

puts b.to_xml

#<?xml version="1.0"?>
#<fields>
#  <field>1</field>
#  <field>2</field>
#</fields>


# now, given b, we want to add a root node, e.g.
#
#<?xml version="1.0"?>
#<root>
#  <fields>
#    <field>1</field>
#    <field>2</field>
#  </fields>
#</root>