code: function commentIssue (client, repo, number, comment) {
    debug("commentIssue", repo, number, comment);
  
    return client.commentIssue(_.merge(extractRepo(repo), {
      "number": number,
      "body":   comment
    })).then(function (comment) {
      return {
        "type":   "comment",
        "issue":  number,
        "url":    comment.html_url
      };
    });
  }
docs: String, Number, String ~→ Comment