'Return the CSS index of the chapter with `title`.
Returns `None` if it cannot find such a chapter.'

def _chapter_index(self, title):

    chapter_css = '.chapters>section    h3'
    chapter_titles = self.q(css=chapter_css).map((lambda el: el.text.lower().strip())).results
    try:
        return (chapter_titles.index(title.lower()) + 1)
    except ValueError:
        self.warning("Could    not    find    chapter    '{0}'".format(title))
        return None
