code: function mr(bundler) {
    var prevError = null
    var pending = null
    var buffer = null
  
    build()
    // bundler.on('update', update)
    return handler
  
    /**
     * Build the CSS and pass it to `buffer`.
     *
     * @api private
     */
    function build() {
      const p = pending = new Emitter()
      buffer = bundler.toString()
  
      if (p !== pending) return
      pending.emit('ready', null, pending = false)
    }
  
    /**
     * Response middleware. Sends `buffer` to
     * either `next()` or `res.body()`.
     *
     * @param {Request} req
     * @param {Response} res
     * @param {Function} next
     * @api private
     */
    function handler(req, res, next) {
      next = next || send
  
      if (pending) return pending.once('ready', function(err) {
        if (err) return next(err)
        return handler(req, res, next)
      })
  
      if (prevError) return next(prevError)
  
      res.setHeader('content-type', 'text/css')
      return next(null, buffer)
  
      function send(err, body) {
        if (err) return res.emit(err)
        res.end(body)
      }
    }
  }
docs: Respond myth in an http request.
  
  @param {Function} bundler
  @return {Function}
  @api public