## in my controller's namespace
  module Responder
    def respond_to
      yield response = Response.new(env)
      response.body
    end

    class Response
      attr_reader :body, :env
      def initialize(env) @env = env end

      TYPES = {
        :yaml => %w[application/yaml text/yaml],
        :text => %w[text/plain],
        :html => %w[*/* text/html application/html]
      }

      def method_missing(method, *args)
        if TYPES[method] && env.HTTP_ACCEPT =~ Regexp.union(*TYPES[method])
          @body = yield if block_given?
        end
      end
    end
  end


## now my actual class
  class Diff < R '/d/(\w+)/(\d+)', '/d/(\w+)/(\d+)/(\d+)'
    include Responder

    def get(title, old_version, new_version = nil)
      @sheet = Sheet.find_by_title(title)
      old_sheet = @sheet.find_version(old_version)
      new_sheet = (new_version ? @sheet.find_version(new_version) : @sheet)

      @diffed = Diffr.diff(old_sheet, new_sheet) rescue nil

      respond_to do |wants|
        wants.html { render :diff }
        wants.yaml { @diffed.to_yaml }
      end
    end
  end