code: function _get_annotation_by_id(aid){
  
      var anchor = this;
      var ret = null;
      each(anchor._annotations, function(ann){
  	if( ann.id() === aid ){
  	    ret = ann;
  	}
      });
      return ret;
  }
docs: Get sublist of annotations with a certain ID.
  
  @name get_annotations_by_id
  @function
  @param {String} aid - annotation ID to look for
  @returns {Array} list of list of annotations with that ID