'Return the CSS to click for `tab_name`.
If no tabs exist for that name, return `None`.'

def _tab_css(self, tab_name):

    all_tabs = self.tab_names
    try:
        tab_index = all_tabs.index(tab_name)
    except ValueError:
        return None
    else:
        return 'ol.course-tabs    li:nth-of-type({0})    a'.format((tab_index + 1))
