anchor.diff:  This function takes the ID of some annotation that already
   exists in the database, adds the new annotation link to the
   currently selected text, and then highlights the lines.
  -
  -Not supported for image annotations.
   %>
   function add_existing_annotation(annotation_text_id) {
     if(is_image) {
  
positive.code: var annotation_manager = null;
  var syntax_highlighter_adapter = null;
  
  var is_image;
  
  function source_code_ready() {
  
negative.code:   return {line_start: line_start, line_end: line_end};
  }
  
  function make_new_annotation() {
    //check if image is shown, and call the appropriate function if so
    if (!is_image) {
      positions = get_mouse_positions();
    } else {
  