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
    klassname = "::" + (self.class.name.split("::")[0..-2] + ["Document"]).join("::")
    klass = begin
              Object.const_get(klassname)
            rescue NameError
              Nokogiri::XML::Document
            end
    @parent = @doc = klass.new
  end
  @context = nil
  @arity = nil
  @ns = nil
  options.each do |k, v|
    @doc.send(:"#{k}=", v)
  end
  return unless block_given?
  @arity = block.arity
  if @arity <= 0
    @context = eval("self", block.binding)
    instance_eval(&block)
  else
    yield self
  end
  @parent = @doc
end