code: function subscribe(prop, notification) {
      if (!notification || typeof notification !== 'function') {
        throw new Error('invalid notification callback');
      }
      return _traverse(_parseProp(prop), null, notification);
    }
docs: Subscribe on changes. Note that object considered changed if anything in subtree has changed. Also, if property path is inside object that was updated, the change is notified.s prop - dotted property path specifies object that triggers notification. notification - called if object under property path changed. Return - notification handler. Used to stop notification calls (closing the handler).