class Nokogiri::XML::Builder

def initialize(options = {}, root = nil, &block)

end
...
Nokogiri::XML::Builder.new(:encoding => 'UTF-8') do |xml|

Building a document with a particular encoding for example:

Document that is being built.
Create a new Builder object. +options+ are sent to the top level
## 
def initialize(options = {}, root = nil, &block)
  if root
    @doc = root.document
    @parent = root
  else
    @parent = @doc = related_class("Document").new
  end
  @context = nil
  @arity = nil
  @ns = nil
  options = DEFAULT_DOCUMENT_OPTIONS.merge(options)
  options.each do |k, v|
    @doc.send(:"#{k}=", v)
  end
  return unless block
  @arity = block.arity
  if @arity <= 0
    @context = eval("self", block.binding)
    instance_eval(&block)
  else
    yield self
  end
  @parent = @doc
end