code: function _get_annotations_by_key(key){
      var anchor = this;
  
      var ret = [];
      each(anchor._annotations, function(ann){
  	if( ann.key() === key ){
  	    ret.push(ann);
  	}
      });
  
      return ret;
  }
docs: Get sublist of annotations with a certain key.
  
  @name get_annotations_by_key
  @function
  @param {String} key - key to look for.
  @returns {Array} list of list of annotations with that key