'Return the number of subsections.
Arguments:
section_title: The section for which to return the number of
subsections. If None, default to the first section.'

@property
def num_subsections(self, section_title=None):

    if section_title:
        section_index = self._section_title_to_index(section_title)
        if (not section_index):
            return
    else:
        section_index = 1
    return len(self.q(css=self.SUBSECTION_TITLES_SELECTOR.format(section_index)))
