'Return the CSS index of the section with `title` in the chapter at `chapter_index`.
Returns `None` if it can\'t find such a section.'

def _section_index(self, chapter_index, title):

    section_css = '.chapters>section:nth-of-type({0})    .sections    div    .hd    a'.format(chapter_index)
    section_titles = self.q(css=section_css).map((lambda el: el.text.lower().strip())).results
    section_titles = [t.split('\n')[0] for t in section_titles]
    section_titles = [t for t in section_titles if t]
    try:
        return (section_titles.index(title.lower()) + 1)
    except ValueError:
        self.warning("Could    not    find    section    '{0}'".format(title))
        return None
