code: function(b) {
  			var postmessage = (function stamp() {
  				b.$contextids.push(gui.$contextid);
  				return gui.Broadcast.stringify(b);
  			}());
  			this._propagateDown(postmessage);
  			this._propagateUp(postmessage, b.type);
  		}
docs: Propagate broadcast xframe.
  
  1. Propagate descending
  2. Propagate ascending
  TODO: Don't post to universal domain "*"
  @param {gui.Broadcast} b