code: function addUrl(when, feed) {
    if (!types.check(feed, 'feed') && !types.check(feed, ['feed']))
      throw Error('sandcrawler.spider.url(s): wrong argument.');
  
    var a = !(feed instanceof Array) ? [feed] : feed,
        c = !this.state.running ? this.initialBuffer.length : this.index,
        job,
        i,
        l;
  
    for (i = 0, l = a.length; i < l; i++) {
      job = createJob(a[i]);
  
      // Don't add if already at limit
      if (this.options.limit && c >= this.options.limit)
        break;
  
      if (!this.state.running) {
        this.initialBuffer[when === 'later' ? 'push' : 'unshift'](job);
      }
      else {
        this.queue[when === 'later' ? 'push' : 'unshift'](job);
        this.emit('job:add', job);
      }
    }
  
    return this;
  }
docs: Feeding the spider