'Show books with only one format'

def marked(self):

    db = self.db.new_api
    matched_ids = {book_id for book_id in db.all_book_ids() if (len(db.formats(book_id)) == 1)}
    self.db.set_marked_ids(matched_ids)
    self.gui.search.setEditText(u'marked:true')
    self.gui.search.do_search()
