Log in as Manager::

    >>> browser.login(TEST_USER_NAME, TEST_USER_PASSWORD)

Go to edit cart container::

    >>> browser.open('/cart-container/edit')
    >>> browser.getControl(name="form.widgets.next_cart_id").value = '2'
    >>> browser.getControl(name="form.buttons.save").click()
    >>> print browser.bycss('#next-cart-id')
    --- 1 ---
    <div class="documentDescription" id="next-cart-id">The next cart ID: 2</div>

Go to edit cart container for the validation::

    >>> browser.open('/cart-container/edit')
    >>> browser.getControl(name="form.widgets.next_cart_id").value = '1'
    >>> browser.getControl(name="form.buttons.save").click()

    >>> browser.url
    'http://nohost/plone/cart-container/@@edit'

    >>> print browser.bycss('.error')
    --- 1 ---
    <dl class="portalMessage error"><dt>Error</dt>
    <dd>There were some errors.</dd>
    </dl>
    --- 2 ---
    <div class="field z3cformInlineValidation kssattr-fieldname-form.widgets.next_cart_id error" id="formfield-form-widgets-next_cart_id">
    <label for="form-widgets-next_cart_id" class="horizontal">
    Next Cart ID
    <span class="required horizontal" title="Required">&#160;</span>
    </label>
    <div class="fieldErrorBox">
    <div class="error">The cart ID is already in use.</div>
    </div>
    ...
