class CardGroup extends Component {
      render() {
        return (
          <div>{this.props.children}</div>
        )
      }
    }
    CardGroup.propTypes = {
      children: function (props, propName, componentName) {
        const prop = props[propName]

        let error = null
        React.Children.forEach(prop, function (child) {
          if (child.type !== Card) {
            error = new Error('`' + componentName + '` children should be of type `Card`.');
          }
        })
        return error
      }
    }