class Kramdown::Document

def method_missing(id, *attr, &block)

For example, +to_html+ would instantiate the Kramdown::Converter::Html class.

class (i.e. a class in the Kramdown::Converter module) and use it for converting the document.
Check if a method is invoked that begins with +to_+ and if so, try to instantiate a converter 
def method_missing(id, *attr, &block)
  if id.to_s =~ /^to_(\w+)$/ && (name = Utils.camelize($1)) &&
      try_require('converter', name) && Converter.const_defined?(name)
    output, warnings = Converter.const_get(name).convert(@root, @options)
    @warnings.concat(warnings)
    output
  else
    super
  end
end