'Return a `Promise` that is fulfilled when the user is on
the correct section and subsection.'

def _on_section_promise(self, section_title, subsection_title):

    desc = "currently    at    section    '{0}'    and    subsection    '{1}'".format(section_title, subsection_title)
    return EmptyPromise((lambda : self.is_on_section(section_title, subsection_title)), desc)
