Look for a form on the page and submit it.

    Asserts if more than one form exists.

def submit_the_only_form(self):
    """
    Look for a form on the page and submit it.

    Asserts if more than one form exists.
    """
    form = ElementSelector(world.browser, str('//form'))
    assert form, "Cannot find a form on the page."
    form.submit()