code: function objectObserver(changes) {
      changes.forEach(({type, name}) => {
        switch (type) {
        case 'add':    o[name] = u[name]; break;
        case 'delete': delete o[name];    break;
        }
      });
    }
docs: Observer to handle new or deleted properties on the object. Pass through to underlying object, which will cause the right things to happen.