code: def page(num, opts = {})
        per_page = opts[:per_page].to_i || 20
        start  = per_page * (num - 1)
        self.class.new(@backend, @initial_key, @associated_class,
          @callback_target_class, @callback_target_id, @callbacks, @sort_order,
          self, ::Zermelo::Filters::Steps::ListStep.new({:offset => start,
            :limit => per_page}, {}))
      end
docs: (a different syntax to the above)