code: public void addComment(Comment comment) {
          assertNotNull(comment);
          if (comments == null) {
              comments = new TreeSet<Comment>(new AstNode.PositionComparator());
          }
          comments.add(comment);
          comment.setParent(this);
      }
docs: Add a comment to the comment set.
  @param comment the comment node.
  @throws IllegalArgumentException if comment is {@code null}