Return list of slugs that correspond to pages to generate.

def _pages_to_generate(self):
        '''Return list of slugs that correspond to pages to generate.'''
        # right now it gets all the files. In theory, It should only
        # get what's changed... but the program is not doing that yet.

        all_pages = self.get_page_names()
        # keep only those whose status is published
        ptg = []
        for slug in all_pages:
            p = s2page.Page(self, slug, isslug=True)
            if p.published:
                ptg.append({'slug': p.slug, 'title':p.title, 'date': p.creation_date })

        # sort the ptg array in reverse chronological order of its entries.
        sptg = sorted(ptg, key=lambda x : x['date'],reverse=True)
        res = [ pinfo['slug'] for pinfo in sptg]
        return res