code: func (d *Downloader) Start() {
  	d.mu.Lock()
  	defer d.mu.Unlock()
  	if d.started {
  		return
  	}
  	d.started = true
  
  	d.pool = common.NewGoroutinePriorityPool(d.ctx, d.options.MaxConcurrentJobs)
  
  	if err := d.ensureDir(d.outputDir); err != nil {
  		d.addError(isolatedType, "<isolated setup>", err)
  		d.cancel()
  		return
  	}
  
  	// Start downloading the isolated tree in the work pool.
  	d.scheduleIsolatedJob(d.rootHash)
  }
docs: // Start begins downloading the isolated.