code: function nextPage(callback) {
      var config;
      var service = this.request.service;
      var operation = this.request.operation;
      try {
        config = service.paginationConfig(operation, true);
      } catch (e) { this.error = e; }
  
      if (!this.hasNextPage()) {
        if (callback) callback(this.error, null);
        else if (this.error) throw this.error;
        return null;
      }
  
      var params = AWS.util.copy(this.request.params);
      if (!this.nextPageTokens) {
        return callback ? callback(null, null) : null;
      } else {
        var inputTokens = config.inputToken;
        if (typeof inputTokens === 'string') inputTokens = [inputTokens];
        for (var i = 0; i < inputTokens.length; i++) {
          params[inputTokens[i]] = this.nextPageTokens[i];
        }
        return service.makeRequest(this.request.operation, params, callback);
      }
    }
docs: Creates a new request for the next page of response data, calling the
  callback with the page data if a callback is provided.
  
  @callback callback function(err, data)
  Called when a page of data is returned from the next request.
  
  @param err [Error] an error object, if an error occurred in the request
  @param data [Object] the next page of data, or null, if there are no
  more pages left.
  @return [AWS.Request] the request object for the next page of data
  @return [null] if no callback is provided and there are no pages left
  to retrieve.
  @since v1.4.0