code: function unshiftArticle(summary, article) {
      article = SummaryArticle(article);
  
      var parts = summary.getParts();
      var part = parts.get(0) || SummaryPart();
  
      var articles = part.getArticles();
      articles = articles.unshift(article);
      part = part.set('articles', articles);
  
      parts = parts.set(0, part);
      summary = summary.set('parts', parts);
  
      return indexLevels(summary);
  }
docs: Insert an article at the beginning of summary
  
  @param {Summary} summary
  @param {Article} article
  @return {Summary}