code: function _get_annotations_by_filter(filter){
  
      var anchor = this;
      var ret = [];
      each(anchor._annotations, function(ann){
  	var res = filter(ann);
  	if( res && res === true ){
  	    ret.push(ann);
  	}
      });
      return ret;
  }
docs: Get a sublist of annotation using the filter function. The filter
  function take a single annotation as an argument, and adds to the
  return list if it evaluates to true.
  
  @name get_annotations_by_filter
  @function
  @param {Function} filter - function described above
  @returns {Array} list of passing annotations