class Nokogiri::XML::Builder

def [] ns

ArgumentError if +ns+ has not been defined higher in the tree.
Build a tag that is associated with namespace +ns+. Raises an
## 
def [] ns
  @ns = @parent.namespace_definitions.find { |x| x.prefix == ns.to_s }
  return self if @ns
  @parent.ancestors.each do |a|
    next if a == doc
    @ns = a.namespace_definitions.find { |x| x.prefix == ns.to_s }
    return self if @ns
  end
  raise ArgumentError, "Namespace #{ns} has not been defined"
end